Furnace Blowing Cold Air: How to Fix It

Last updated: April 2026

By the HVAC Pricing Guide Team

This is usually one of three things, and two of them you can fix yourself in the next 15 minutes. Check your thermostat fan setting first, then the air filter, then the pilot light or igniter. This guide covers every reason a furnace blows cold air, organized from the simplest DIY fixes to the problems that need a technician.

For complete furnace repair pricing, see our furnace repair cost guide. For an interactive step-by-step diagnosis tool, try our HVAC troubleshooter.

What Should You Check First?

Three checks resolve the majority of "furnace blowing cold air" calls without a technician. Work through these in order before calling anyone.

Check 1: Thermostat fan setting (ON vs AUTO)

This is the number one cause of "my furnace is blowing cold air" calls, and it is not a malfunction at all. Look at your thermostat and find the FAN setting. It will be set to either ON or AUTO. If it is set to ON, the blower motor (the fan inside the furnace that pushes air through the ductwork) runs continuously, 24 hours a day, regardless of whether the burners are firing. During the periods between heating cycles, the blower pushes room-temperature air through the ductwork. Because this air has not been heated, it feels noticeably cold coming from the vents, especially in winter when duct runs pass through unconditioned spaces like the attic, crawl space, or unheated basement where the air in the ducts cools below room temperature.

Switch the fan setting to AUTO. In AUTO mode, the blower only runs when the burners are actively firing and the heat exchanger (the metal chamber inside the furnace where combustion heat transfers to the household air passing over it) has reached operating temperature. The blower starts 30 to 90 seconds after the burners ignite and runs for 60 to 120 seconds after the burners shut off to extract residual heat from the heat exchanger. Between heating cycles, the blower is off and no air comes from the vents. If switching from ON to AUTO solves the cold air problem, you are done. There was never a malfunction.

Some homeowners set the fan to ON intentionally to circulate air continuously for even temperature distribution or air filtration. If you prefer continuous fan operation, understand that the air from the vents will feel cool between heating cycles. This is normal operation, not a problem with the furnace.

Check 2: Air filter

A clogged air filter is the second most common cause of a furnace blowing cold air, and unlike the fan setting, this one is an actual problem that causes the furnace to shut down as a safety measure. The air filter is located inside the return air grille on a wall or ceiling, or in the slot where the large return duct connects to the furnace cabinet. Pull the filter out and hold it up to a light source. If you cannot see light through the filter material, it is severely clogged and needs to be replaced immediately.

Here is what happens when a filter is clogged: the restricted airflow reduces the volume of air passing over the heat exchanger. With less air absorbing the heat, the heat exchanger temperature rises above its design limit. The high-limit switch (a safety device mounted on the heat exchanger that monitors its temperature) detects the overtemperature condition and shuts off the gas valve and burners to prevent the heat exchanger from cracking. A cracked heat exchanger can leak carbon monoxide (a colorless, odorless, toxic gas produced by combustion) into the household air stream, which is why the safety switch exists. After the burners shut off, the blower continues running to cool the heat exchanger back down to a safe temperature. During this cooling period, the blower pushes unheated air through the vents. Once the heat exchanger cools sufficiently, the high-limit switch resets, the burners reignite, the heat exchanger overheats again, and the cycle repeats. This creates a pattern of brief warm air followed by extended cold air that is the hallmark of a clogged filter problem.

Replace the clogged filter with the same size (the dimensions are printed on the filter frame). Standard 1-inch filters cost $5 to $15. After replacing the filter, give the furnace 15 to 30 minutes to stabilize and run through a few complete heating cycles. If the furnace produces consistent heat without cycling the burners on and off rapidly, the clogged filter was the cause. Going forward, check the filter monthly during heating season and replace it when it is visibly dirty. Most homes need a new filter every 30 to 90 days depending on the filter type, household size, pets, and dust levels.

Check 3: Pilot light or igniter

If the fan is set to AUTO and the filter is clean but the furnace is still blowing cold air, the burners may not be lighting at all. What you will observe is the blower running for a minute or two and then shutting off, then the sequence starting again. This happens because the furnace control board sends a signal to ignite, the igniter fails to light the gas, the control board times out and shuts down the ignition attempt, and the sequence retries (typically 3 times before locking out). The approach to checking this depends on whether your furnace uses a standing pilot light or electronic ignition, which is covered in the next two sections. First, determine which type you have. If your furnace was manufactured before approximately 2000, it likely has a standing pilot light (a small continuously burning flame). If it was manufactured after 2000, it almost certainly has electronic ignition (either a hot surface igniter or a spark igniter). You can also look through the small window on the furnace front panel: if you see a small blue flame burning continuously, that is a standing pilot.

How Do You Fix a Gas Furnace with a Standing Pilot Light?

Gas furnaces with standing pilot lights use a small continuously burning flame to ignite the main burners when the thermostat calls for heat. If the pilot light goes out, the main burners cannot ignite, and the furnace blows unheated air or fails to start entirely. Standing pilot systems are common in furnaces manufactured before 2000, though some budget models continued using them into the mid-2000s.

How to relight the pilot light

Relighting a pilot light is a straightforward procedure that most homeowners can do safely. Locate the gas valve on the furnace. This is a box-shaped component on the gas pipe that enters the furnace, with a knob or dial that has three positions: ON, OFF, and PILOT. Turn the knob to OFF and wait a full 5 minutes. This waiting period allows any accumulated gas to dissipate. If you smell gas strongly during this wait, do not proceed. Leave the house and call your gas utility from outside. See the carbon monoxide and gas safety section below.

After waiting 5 minutes with no gas smell, turn the knob to PILOT. Press and hold the knob in (or press and hold a separate red reset button, depending on the model). While holding the knob or button, use a long-reach lighter or long fireplace match to bring a flame to the pilot burner opening. The pilot opening is a small metal tube located near the main burner assembly, usually visible through an access window on the lower front panel of the furnace. Once the pilot flame appears (a small, steady, blue flame), continue holding the knob or button for 30 to 60 seconds. This heating period allows the thermocouple (a safety sensor positioned in the pilot flame that generates a small electrical signal when heated) to warm up and signal the gas valve that a flame is present.

Release the knob or button. The pilot flame should remain burning on its own. If it does, turn the gas valve knob from PILOT to ON. The furnace should now respond to thermostat calls for heat by igniting the main burners from the pilot flame. If the pilot flame goes out when you release the knob, the thermocouple is likely faulty and needs replacement.

When the pilot will not stay lit: thermocouple failure

The thermocouple is a thin copper tube that extends from the gas valve to the tip of the pilot flame. When the pilot flame heats the thermocouple tip, it generates a small electrical voltage (measured in millivolts) that tells the gas valve to remain open. When the thermocouple fails (from corrosion, physical damage, or age-related degradation), it cannot generate enough voltage to keep the gas valve open, so the valve closes and the pilot goes out as soon as you release the reset button.

A thermocouple replacement is one of the least expensive furnace repairs: $100 to $250 including parts and labor. The thermocouple itself costs $10 to $25 as a part and is available at hardware stores. The replacement involves disconnecting the old thermocouple from the gas valve (it threads off with a wrench), removing it from the pilot assembly bracket, positioning the new thermocouple so its tip sits directly in the pilot flame, and threading the connector back onto the gas valve. While this is technically a DIY repair, if you are not comfortable working near gas components, a technician can complete it in 30 to 45 minutes.

Pilot flame color and what it means

A healthy pilot flame is blue with a small yellow tip, steady and not flickering, and about 1 to 2 inches tall. The flame should engulf the top third of the thermocouple tip. If the pilot flame is mostly yellow, wavering or flickering excessively, or very small and weak, the pilot assembly may be dirty (dust or debris partially blocking the gas orifice), the gas pressure may be too low, or there may be a draft blowing across the pilot. A yellow, lazy pilot flame produces less heat, may not keep the thermocouple hot enough to hold the gas valve open, and indicates incomplete combustion. A technician can clean the pilot assembly and adjust the gas pressure if needed, typically as part of a standard service call ($75 to $150).

How Do You Fix a Gas Furnace with Electronic Ignition?

Most furnaces manufactured after 2000 use electronic ignition instead of a standing pilot. There are two types: hot surface ignition (HSI), which uses a glowing element similar to a car's cigarette lighter to ignite the gas, and intermittent pilot ignition (IPI), which uses an electric spark to light a pilot flame that then lights the main burners. Hot surface ignition is far more common in residential furnaces.

Understanding error codes on the control board

Modern furnaces with electronic ignition have a control board (a circuit board inside the furnace that manages the ignition sequence, blower operation, and safety monitoring) with an LED indicator light. This LED flashes a pattern that corresponds to a specific diagnostic code. To read the code, find the small viewing window on the lower front panel of the furnace and count the number of flashes. The LED flashes a certain number of times, pauses, then repeats the same pattern. Compare the flash count to the diagnostic chart printed on the inside of the furnace access panel door (you may need to remove the upper and lower panels to find it). Common flash codes and their meanings are listed below.

Note that flash codes vary by manufacturer. The chart on your furnace panel is the definitive reference for your specific model. If the error code persists after addressing the indicated cause (replacing the filter for a code 4, for example), the control board may need to be reset. Turn the furnace power switch off (a light switch usually mounted on the side of the furnace or on the wall nearby), wait 30 seconds, then turn it back on. This resets the control board and allows the ignition sequence to attempt again. If the same error code returns after 3 ignition attempts, the underlying cause has not been resolved and a technician is needed.

Flame sensor cleaning (a common DIY fix)

The flame sensor is one of the most frequent causes of a gas furnace failing to produce heat, and cleaning it is a task many homeowners can do themselves. The flame sensor is a thin metal rod (typically 2 to 4 inches long, made of stainless steel or nickel alloy) that extends into the path of the burner flame. Its job is to detect that the gas has actually ignited after the gas valve opens. When the sensor becomes coated with carbon buildup, oxidation, or silicon deposits (from certain types of air fresheners, cleaning products, and personal care products that contain silicone), it cannot detect the flame. The control board sees no flame signal within a few seconds of opening the gas valve, assumes the gas did not ignite, and shuts the gas valve as a safety precaution. The blower may run briefly to purge any unburned gas.

To clean the flame sensor, first turn off power to the furnace at the power switch or breaker. Remove the furnace access panel (usually the lower panel). Locate the flame sensor, which is a thin rod mounted with a single screw near the burner assembly, with a single wire connected to it. Remove the screw, gently slide the sensor out, and disconnect the wire. Use fine-grit sandpaper (220 grit works well) or a fine abrasive pad to gently rub the metal rod until the surface is clean and shiny. Some technicians use the slightly abrasive paper of a dollar bill for light cleaning. Do not use coarse sandpaper, steel wool, or aggressive abrasives that could damage the sensor surface. Reattach the sensor with the mounting screw, reconnect the wire, replace the access panel, and restore power. Run the furnace through a heating cycle to verify the burners light and stay lit.

A flame sensor that needs cleaning is usually the cause when a furnace lights briefly (you see the burners ignite through the view window) and then shuts down within 3 to 10 seconds. If cleaning does not resolve the issue, the flame sensor itself may be cracked or have an open circuit, and needs replacement ($75 to $200 for parts and labor). Flame sensor cleaning should be part of every annual furnace maintenance visit.

Cracked igniter

The hot surface igniter is a fragile element (usually made of silicon carbide or silicon nitride) that glows orange-red when electricity passes through it, reaching 1,800 to 2,500 degrees Fahrenheit to ignite the gas. Igniters crack from thermal cycling (heating to extreme temperature and cooling repeatedly), physical vibration, or handling during maintenance. A cracked igniter may glow but not reach ignition temperature, or may not glow at all. When the igniter fails to reach temperature within the control board's timing window (usually 15 to 60 seconds), the ignition sequence aborts.

You can visually inspect the igniter by turning off power to the furnace, removing the access panel, and looking at the igniter element. If you see a visible crack through the element, the igniter needs replacement. Igniter replacement costs $200 to $400 including parts and labor. The part itself costs $20 to $75 depending on the furnace model. The repair involves disconnecting the wire harness, removing one or two mounting screws, installing the new igniter without touching the element with bare fingers (skin oils shorten the element lifespan), and testing the system through multiple cycles. While technically a DIY repair, the fragility of the part and the need to handle it without contamination make professional installation a reasonable choice.

What Causes an Electric Furnace to Blow Cold Air?

Electric furnaces use heating elements (coils of high-resistance wire that glow red-hot when electricity passes through them, similar to a toaster) instead of gas burners to produce heat. They are less common than gas furnaces in most of the country but are standard in areas without natural gas service and in many apartment and condominium complexes. Electric furnace heating failures have different causes than gas furnace failures.

Tripped breaker or blown fuse

Electric furnaces draw significant electrical current, typically on two separate 240-volt breakers in the electrical panel. If one breaker trips, the furnace may still run the blower but only half of the heating elements receive power, resulting in air that feels warm but not hot. If both breakers trip, the blower may still operate on its own circuit while no heating elements function. Check the electrical panel for tripped breakers. Reset by flipping the breaker fully to OFF, then back to ON. If the breaker trips again, there is an electrical fault that requires a technician.

Failed sequencer

The sequencer is a device that turns on the heating elements in stages rather than all at once. Activating all elements simultaneously would create an enormous instantaneous electrical demand that could trip breakers and stress the electrical system. The sequencer energizes the first set of elements, waits 10 to 30 seconds, energizes the second set, and so on. When a sequencer fails, the heating elements it controls do not activate, and the furnace produces reduced heat or no heat depending on how many sequencers have failed (larger electric furnaces have multiple sequencers). Sequencer replacement costs $150 to $350 including parts and labor.

Failed heating elements

Individual heating elements can burn out over time, especially in furnaces that are 10 or more years old. When an element fails, the furnace loses that portion of its heating capacity. If one of four elements fails, you get roughly 75% heating capacity, which may be enough to maintain temperature on mild days but not during cold weather. If multiple elements fail, the furnace blows noticeably cool air. A technician tests each element with a multimeter (an electrical measuring device) to identify which ones have failed. Element replacement costs $100 to $300 per element including labor.

Electric furnace troubleshooting summary

Before calling a technician for an electric furnace blowing cold air, check the thermostat settings (mode and temperature), replace the air filter if it is dirty, and check both breakers in the electrical panel. If these three checks do not resolve the issue, a technician needs to test the sequencer, heating elements, and high-limit switch to identify the failed component.

Why Is Your Heat Pump Blowing Cold Air?

Heat pumps work differently from furnaces, and understanding the difference explains why cold air from a heat pump is sometimes normal and sometimes a problem. A heat pump is essentially an air conditioner that can run in reverse. In cooling mode, it absorbs heat from indoor air and releases it outside. In heating mode, it absorbs heat from outdoor air and releases it inside. Because it extracts heat from outdoor air rather than generating it through combustion or electric resistance, the air it produces in heating mode is warm (typically 90 to 105 degrees from the vents) rather than hot (120 to 140 degrees from a gas furnace). Some homeowners accustomed to gas furnace output perceive heat pump supply air as "cold" even when the system is operating normally.

Defrost mode is normal operation

During cold weather (below 40 degrees), moisture from the outdoor air condenses on the outdoor coil and can freeze, forming a layer of frost or ice. To prevent this ice from blocking airflow, the heat pump periodically enters defrost mode. During defrost, the system briefly reverses to cooling mode for 5 to 15 minutes, running the hot refrigerant gas through the outdoor coil to melt the ice. While in defrost mode, the indoor coil is running in cooling mode, and the air from the vents feels cool or lukewarm. This is normal and expected behavior. The outdoor unit may also produce a whooshing sound and visible steam during defrost. The system returns to heating mode automatically once the defrost cycle completes.

Defrost cycles should occur every 30 to 90 minutes during cold weather and last no more than 15 minutes. If the system seems to be in defrost mode constantly, running defrost cycles every 15 to 20 minutes, or spending more than 15 minutes in defrost, there may be a problem with the defrost control board (which times the defrost cycles), the defrost sensor or thermostat (which measures outdoor coil temperature), or the outdoor coil may be so heavily iced that normal defrost cycles cannot clear it. Our heat pump repair cost guide covers the diagnostic and repair costs for defrost system issues.

Auxiliary heat not engaging

Most heat pump systems include supplemental heating (called auxiliary heat or emergency heat) that activates when the outdoor temperature drops below the heat pump's effective heating range (typically below 30 to 35 degrees, though this varies by model and climate). Auxiliary heat is usually an electric resistance heating strip (similar to an electric furnace element) installed in the air handler. When auxiliary heat fails to engage, the heat pump continues operating alone but cannot produce enough heat to maintain the set temperature, and the supply air feels lukewarm or cool. Causes include a failed auxiliary heating element, a failed sequencer that controls the auxiliary elements, a thermostat that is not configured to activate auxiliary heat, or a tripped breaker on the auxiliary heat circuit. A technician checks the auxiliary heat components and thermostat configuration to identify the cause. Auxiliary heat element replacement costs $150 to $400.

Reversing valve stuck

The reversing valve is a component in the outdoor unit that switches the direction of refrigerant flow to change between heating and cooling modes. When the reversing valve sticks in the cooling position, the heat pump runs in cooling mode even when the thermostat calls for heat, producing cold air from the vents. A stuck reversing valve may respond to light tapping on the valve body (not recommended as a permanent fix) or may need replacement. Reversing valve replacement is a significant repair costing $400 to $900 including parts and labor, because it requires recovering refrigerant, brazing the new valve into the refrigerant circuit, and recharging the system.

Low refrigerant in heating mode

Just as low refrigerant reduces cooling capacity in summer, it reduces heating capacity in winter. A heat pump with a refrigerant leak produces supply air that is barely warm and runs constantly without reaching the set temperature. The same diagnostic and repair process applies as for cooling: a technician locates the leak, repairs it, and recharges the system. Heating mode refrigerant leak repair costs $200 to $1,500 for the leak repair plus $200 to $700 for the refrigerant recharge.

How Do You Diagnose "Running but No Heat"?

If you have completed the three initial checks (fan setting, filter, pilot/igniter) and the furnace is still blowing cold air, work through this diagnosis tree to narrow down the cause before calling a technician. This information also helps you have an informed conversation with the technician and evaluate their diagnosis.

Step 1: Is the blower running?

If yes, air is moving through the vents but it is not warm. Proceed to Step 2. If no, the entire system is not running. Check the thermostat settings, the furnace power switch (usually on the side of the furnace or on a nearby wall), and the circuit breaker. If all three are correct and the furnace still does not start, the control board, transformer, or thermostat wiring may have failed.

Step 2: Do the burners ignite at all?

Look through the view window on the furnace front panel during a heating call. You should see the igniter glow (orange, on electronic ignition furnaces), then the burner flame appear (blue flames, sometimes with small yellow tips). If the igniter glows but the burners do not light, the gas valve may not be opening, the gas supply may be shut off (check the manual shutoff valve on the gas line near the furnace), or the gas pressure may be insufficient. If the igniter does not glow, the igniter has failed or the control board is not sending power to it. If the burners light and then go out within 3 to 10 seconds, the flame sensor is dirty or failed (see the flame sensor cleaning section above).

Step 3: Do the burners stay lit for the full heating cycle?

If the burners light and stay lit for a few minutes but then shut off while the blower continues running, the high-limit switch is tripping due to overheating. The most common cause is a clogged filter (go back and double-check the filter). Other causes include a failed blower motor (the blower is not moving enough air), a blocked return air path (furniture or objects covering the return grille), or a dirty evaporator coil (in systems with AC and heating sharing the same air handler) restricting airflow. If the filter is clean and the return air path is clear, a technician needs to check the blower motor speed and the heat exchanger condition.

Step 4: Is the inducer motor running?

Before the burners ignite, the draft inducer motor (a small motor on the side of the furnace that vents combustion gases through the flue pipe) must start and run long enough to activate the pressure switch (a safety device that confirms the inducer is creating proper draft). If the inducer motor does not start, you will hear nothing from the furnace during the startup sequence. If the inducer runs but the pressure switch does not close (indicated by a 3-flash error code on many furnaces), the exhaust vent may be blocked (check the vent pipe where it exits the house for bird nests, ice, or debris), the inducer motor may be weak, or the pressure switch itself may have failed. Inducer motor replacement costs $300 to $700. Pressure switch replacement costs $75 to $200.

Why Is a Clogged Filter the Number One Cause of No Heat Calls?

HVAC technicians consistently report that clogged air filters account for 25 to 40% of all "furnace not heating" service calls. Understanding why the filter has such a dramatic impact on furnace operation helps explain why this simple, inexpensive maintenance item causes the most expensive-seeming symptoms.

The airflow and heat exchanger relationship

The furnace heat exchanger is designed to transfer a specific amount of heat to a specific volume of air passing over it per minute. When the air filter clogs, the volume of air drops. The same amount of heat is being produced by the burners (the gas flow rate does not change), but less air is there to absorb it. The heat exchanger temperature rises. The high-limit safety switch detects the overtemperature and shuts down the burners. The blower continues running to cool the heat exchanger. Once cooled, the burners reignite, and the cycle repeats. This creates the characteristic symptom pattern: a few minutes of warm air, then cold air, then warm again, then cold again.

The long-term damage from ignoring it

Repeatedly tripping the high-limit switch does more than cause discomfort. Each overheating cycle stresses the heat exchanger through thermal expansion and contraction. Over time, this repeated stress can develop cracks in the heat exchanger, which is both a safety hazard (cracked heat exchangers can leak carbon monoxide into the air stream) and an expensive repair. Heat exchanger replacement costs $1,500 to $3,500, and many technicians recommend replacing the entire furnace rather than just the heat exchanger because the labor cost is similar. All of this from a $10 filter that was not changed on time.

Filter replacement schedule

Standard 1-inch filters should be checked monthly and replaced every 30 to 90 days during the heating season. Pleated filters (the accordion-style type with more surface area) last longer than flat fiberglass filters. Homes with pets, multiple occupants, or construction activity nearby need more frequent replacement. Four-inch media filters used in larger filter cabinets can last 6 to 12 months but should still be checked quarterly. Set a reminder on your phone or calendar for monthly filter checks. The 2 minutes it takes to check the filter can prevent hundreds of dollars in repair costs.

When Should You Shut the Furnace Off Immediately?

A furnace blowing cold air is usually not dangerous because the safety switches are working as designed. However, certain symptoms indicate a condition where continued operation poses a safety risk. Turn the furnace off at the power switch and (for gas furnaces) turn the gas valve to OFF if any of the following are present.

Gas smell

Natural gas is odorless in its natural state, so utilities add a chemical called mercaptan that produces a distinctive rotten egg or sulfur-like smell. If you smell this odor near the furnace, near any gas line in the house, or throughout the house, turn the furnace off immediately. Do not flip any electrical switches (including light switches), do not use your phone inside the house, and do not start any vehicles in an attached garage. Leave the house with all household members and pets, leaving the door open behind you. From outside the house, call your gas utility emergency line and 911. Do not re-enter the house until the gas utility has confirmed it is safe. Gas leaks can cause explosions and are always treated as emergencies.

Burning smell

A brief burning smell when the furnace first starts for the season is normal (dust burning off the heat exchanger and burners). A burning smell that appears during the heating season, persists for more than 15 to 20 minutes, or has an acrid or electrical quality indicates an overheating component. Possible causes include an overheating blower motor, melting wire insulation, a cracked heat exchanger allowing combustion gases to enter the air stream, or something that has fallen onto the heat exchanger through the duct system. Turn the system off and call for service.

Banging noises

A loud bang when the burners ignite (called delayed ignition) indicates that gas is accumulating in the combustion chamber before the igniter lights it. When the accumulated gas finally ignites, it does so all at once with a concussive force that can crack the heat exchanger, blow open the exhaust vent joints, and damage the burner assembly. Delayed ignition is caused by dirty burners, a misaligned igniter, or low gas pressure. If you hear a bang when the furnace starts, turn it off and call a technician. Do not continue to operate the furnace until the delayed ignition is corrected.

Water leaking from the furnace

High-efficiency condensing furnaces (rated at 90% AFUE or higher, identifiable by a PVC plastic exhaust vent rather than a metal flue pipe) produce condensation as a normal part of operation. This condensation drains through a tube to a floor drain or condensate pump. If the drain clogs or the condensate trap cracks, water pools around the base of the furnace. While not immediately dangerous, water near electrical components can cause short circuits. Turn the system off and address the drain issue. Standard-efficiency furnaces (metal exhaust flue, 80% AFUE) should not produce visible condensation. Water from a standard furnace may indicate a cracked heat exchanger, which requires immediate professional evaluation.

What Are the Carbon Monoxide Warning Signs?

Carbon monoxide (CO) is a colorless, odorless gas produced by the combustion of natural gas, propane, oil, or any carbon-based fuel. A properly functioning furnace vents all CO through the exhaust flue to the outside. A malfunctioning furnace (cracked heat exchanger, blocked exhaust vent, backdrafting) can release CO into the household air stream, where it is inhaled by the occupants. CO poisoning is responsible for approximately 400 deaths per year in the United States and thousands of emergency room visits.

CO detector placement and response

Every home with a gas furnace, gas water heater, gas stove, or attached garage should have at least one carbon monoxide detector on every level of the home and one near each sleeping area. CO detectors are available for $20 to $50 at hardware stores and should be replaced every 5 to 7 years (check the expiration date on the back). If a CO detector alarms, do not assume it is a false alarm. Open windows, turn off the furnace, leave the house immediately with all household members and pets, and call 911 from outside. Do not re-enter the house until emergency responders have tested the air and confirmed it is safe.

CO poisoning symptoms

Early symptoms of CO poisoning include headache, dizziness, weakness, nausea, vomiting, chest tightness, and confusion. These symptoms are often mistaken for the flu, food poisoning, or general fatigue. A key distinguishing factor: if multiple household members develop the same symptoms at the same time, especially if symptoms improve when you leave the house and return when you come back, suspect CO poisoning. Prolonged exposure to elevated CO levels can cause loss of consciousness, permanent brain damage, and death. If anyone in the household shows symptoms and you suspect CO, get everyone outside immediately and call 911.

How to reduce CO risk

Schedule annual furnace maintenance that includes a combustion analysis (a test where the technician measures CO levels in the flue gas to verify they are within safe limits) and a heat exchanger inspection. Replace CO detectors on schedule. Never block or redirect the furnace exhaust vent. Never operate the furnace with the access panels removed (this disrupts proper combustion air flow). Never run a gas generator, charcoal grill, or camp stove inside the house or in an attached garage, even with the door open. These are the most common sources of residential CO poisoning.

When Should You Call a Professional?

After completing the DIY checks and troubleshooting steps described above, call a professional HVAC technician if any of the following conditions apply.

Pilot light will not stay lit after two attempts

If you have followed the relighting procedure twice and the pilot goes out each time when you release the reset button, the thermocouple or gas valve needs professional attention. Continuing to attempt relighting with accumulated gas is a safety risk. A thermocouple replacement costs $100 to $250 and takes a technician about 30 to 45 minutes.

Error code persists after resetting

If the control board LED shows the same error code after you have addressed the indicated cause and reset the system (power off for 30 seconds, then back on), the underlying component has failed and needs professional diagnosis. Bring the error code number with you when you call to schedule service, as it helps the technician prepare the likely replacement part before the visit.

Multiple ignition cycles without sustained heat

If the furnace attempts ignition (you see the igniter glow and possibly the burners light briefly) but fails to produce sustained heat after 3 or more attempts, the control board locks out the ignition sequence as a safety measure. This lockout typically requires a manual reset (power cycle) and will recur until the underlying cause (dirty flame sensor, weak igniter, gas valve issue) is resolved. A technician identifies and addresses the specific cause.

System is 15 or more years old

Gas furnaces older than 15 years have reached the age where multiple components are approaching end of life. If your furnace is in this age range and stops producing heat, have the technician provide both a repair quote and an assessment of the system's overall condition and remaining useful life. A repair costing more than $500 on a 15-plus year old furnace is often better invested in a new system. New furnaces cost $4,000 to $8,000 installed depending on size, efficiency, and fuel type. See our furnace installation cost guide for complete pricing and our when to replace guide for the repair-versus-replace decision framework.

Any safety concern

Gas smell, persistent burning smell, CO detector alarm, yellow pilot flame, soot around the furnace, water coming from a standard-efficiency (non-condensing) furnace, or repeated breaker trips all warrant professional inspection before the system is operated again. These symptoms may indicate conditions that could cause a fire, explosion, or CO poisoning. The cost of a service call ($75 to $150) is insignificant compared to the safety risk of operating a furnace with an undiagnosed safety issue.

How Much Does It Cost to Fix a Furnace Blowing Cold Air?

The cost to restore heat depends on the cause. Here is what each common repair costs, from the simplest fixes to major component replacements.

The service call fee ($75 to $150) covers the technician's travel and diagnostic time. Some companies apply this fee toward the cost of the repair if you authorize the work. Others charge it as a separate fee. Ask about the fee structure when you schedule the appointment. For a comprehensive overview of all furnace repair costs, see our furnace repair cost guide. For blower motor replacement costs specifically, see our blower motor cost guide.

When repair cost approaches replacement cost

A new gas furnace costs $4,000 to $8,000 installed depending on size, efficiency rating, and brand. If the repair estimate exceeds $800 and the furnace is over 15 years old, get a replacement quote for comparison. The efficiency gain from replacing a 15-plus year old 80% AFUE furnace with a new 96% AFUE furnace can save $200 to $500 per year in gas bills, which offsets the replacement cost over 8 to 15 years. Our HVAC cost calculator estimates replacement costs based on your home's specifics.

Emergency and after-hours repair costs

Furnace failures are more common on the coldest days when the system runs hardest. Emergency service on evenings, weekends, and holidays typically adds $50 to $200 to the standard service call fee, and some companies charge time-and-a-half or double-time labor rates during these periods. If the weather is mild enough that the house will stay above 55 degrees overnight, waiting until regular business hours saves the emergency surcharge. If temperatures could drop below freezing, the risk of frozen pipes ($2,000 to $10,000 in damage) makes the emergency surcharge worthwhile. For emergency repair pricing and guidance, see our emergency HVAC repair guide.

Frequently Asked Questions