AC Capacitor Blown: Symptoms and What to Do
Last updated: April 2026
Your AC is humming from the outdoor unit but the compressor is not starting. This is the single most common symptom of a blown or failing capacitor, and capacitor failure is the single most common AC repair in the United States, accounting for an estimated 30 to 40% of all summer AC service calls. Replacement costs $150 to $400 including parts and labor. A technician can complete the job in 15 to 30 minutes. The important thing to know: a blown capacitor is a straightforward, inexpensive repair, but running the system with a failing capacitor damages the $1,500 to $3,000 compressor. Here is how to identify the problem, what to check safely, and what to do next.
For detailed pricing by capacitor type, see our AC capacitor replacement cost guide. For all AC repair costs, see our national AC repair cost guide. If you are not sure whether your problem is a capacitor or something else, our HVAC troubleshooter walks you through diagnosis step by step.
What Are the Symptoms of a Blown AC Capacitor?
Capacitor failure produces a distinctive set of symptoms that differ from other AC problems. Learning to recognize these symptoms helps you provide the right information when calling a technician, which can save diagnostic time and potentially reduce the service cost.
Symptom 1: AC Hums But Does Not Start (Most Common)
The most recognizable symptom of a blown capacitor is a humming or buzzing sound from the outdoor unit when the thermostat calls for cooling, but the compressor does not start and no air is being cooled. What is happening: the thermostat sends a signal to the outdoor unit, the contactor (the electrical relay switch that sends power to the compressor) closes, power reaches the compressor motor, and the motor attempts to start. Without a working capacitor to provide the starting energy boost, the motor cannot overcome its initial inertia. The motor windings energize and produce a magnetic hum, but the motor shaft does not turn. The system sits there humming until a safety switch (usually a thermal overload on the compressor) detects the overcurrent condition and shuts the circuit off. After a cool-down period, the cycle repeats.
This hum-but-no-start pattern is nearly diagnostic for a capacitor issue. The only other components that can produce this symptom are a seized compressor (the motor cannot physically turn because the internal mechanism is locked, a much more serious and expensive problem at $1,500 to $3,000) or a failed start relay (less common, $100 to $250 to replace).
Symptom 2: Fan Spins Slowly or Not at All
The condenser fan (the fan on top of the outdoor unit) uses the same capacitor or a shared section of a dual capacitor to run. If the fan motor side of the capacitor has failed, the fan may spin very slowly (you can visually see it struggling to maintain speed), spin intermittently, or not spin at all. In some cases, the fan tries to start and makes a stuttering motion before stopping. If the compressor starts but the fan does not spin, the outdoor unit cannot release heat, and the compressor will overheat and shut off within minutes. This creates a short cycling pattern.
A quick test: if the fan is not spinning but the compressor is humming, you can sometimes give the fan blade a gentle spin through the top grate with a long stick or screwdriver (with caution, and never put your hand through the grate while the power is on). If the fan starts spinning and continues after the nudge, the capacitor is almost certainly the problem. The fan motor has enough power to run once spinning but not enough to start from a dead stop without the capacitor boost. This is a common field test technicians use, but do it carefully and only from outside the unit through the top grate.
Symptom 3: System Starts Then Shuts Off Quickly
A capacitor that is failing but not completely dead may provide enough energy to start the compressor, but the weakened capacitor cannot sustain the run current the motor needs. The compressor starts, runs for 30 seconds to a few minutes, and then the motor overheats or loses the electrical support it needs to stay running and shuts off on the thermal overload. This intermittent operation is particularly damaging to the compressor because each failed start cycle stresses the motor windings and generates excess heat without the sustained running that normally cools the motor.
Symptom 4: Clicking Sound Followed by Nothing
When the thermostat calls for cooling, you may hear a click from the outdoor unit (this is the contactor closing, sending power to the compressor circuit), followed by a brief hum that quickly stops, followed by silence. The click means the contactor is working. The brief hum means power is reaching the compressor. The quick stop means the compressor could not start and the safety switch tripped. This click-hum-stop sequence repeated every few minutes is a classic capacitor failure pattern.
Symptom 5: Burning or Chemical Smell Near the Outdoor Unit
A severely failed capacitor may emit a faint burning or chemical odor. This comes from the electrolytic fluid inside the capacitor heating up or leaking when the capacitor's internal insulation breaks down. If you detect this smell, turn the system off at the thermostat and the disconnect switch immediately. The smell does not indicate a fire hazard in most cases, but it confirms a component failure that needs professional repair. Do not confuse this with a persistent burning electrical smell, which could indicate a different and potentially more serious problem like a failing compressor motor or wiring issue.
Symptom 6: Visible Capacitor Damage
If you open the access panel on the outdoor unit (after turning off the power at the disconnect switch), you can visually inspect the capacitor. A healthy capacitor is a smooth cylinder with a flat top. A failed capacitor may show a bulging or domed top (the internal pressure from electrolyte failure pushes the top outward), oily residue or dried fluid around the base or connection terminals, charring or discoloration on the capacitor body, visible cracks in the casing, or a melted or warped plastic insulation sleeve. Any of these visual signs confirms the capacitor has failed. However, some failed capacitors look perfectly normal from the outside and can only be identified as failed through electrical testing with a capacitance meter.
What Does an AC Capacitor Do?
A capacitor is a small cylindrical electrical component, typically 2 to 4 inches tall and 1 to 2 inches in diameter, that stores electrical energy and releases it in bursts. In an AC system, the capacitor serves two critical functions. First, it provides the high starting torque that the compressor motor needs to begin spinning from a dead stop. An electric motor needs 3 to 10 times more energy to start than to keep running. The capacitor stores this energy between cycles and delivers it during startup. Second, the capacitor helps maintain the correct voltage and current phase relationship that the motor needs to run efficiently. This is called power factor correction.
Types of AC Capacitors
Most residential AC systems use one of two capacitor configurations. A dual run capacitor is a single component with three terminals that serves both the compressor motor and the condenser fan motor. This is the most common configuration in modern residential AC units. One terminal is labeled "C" (common), one is labeled "HERM" (hermetic, meaning the compressor), and one is labeled "FAN." When a dual run capacitor fails, it may fail on one side (affecting only the compressor or only the fan) or both sides (affecting everything). Replacement cost: $15 to $35 for the part, $150 to $400 installed.
A start capacitor is a separate component used in systems with a hard-start kit. It provides an extra burst of starting energy for compressors that struggle to start under load (typically older compressors or compressors in extreme heat environments). Start capacitors are only active for 1 to 2 seconds during startup and then disconnect via a relay. They fail less frequently than run capacitors because they are active for only a fraction of each cycle. Replacement cost: $50 to $150 for the part and relay, $200 to $400 installed.
Capacitor Ratings
Every capacitor has two key ratings printed on its label: capacitance (measured in microfarads, abbreviated MFD or uF) and voltage rating. Common residential dual run capacitor ratings include 35/5 MFD at 370V or 440V (35 microfarads for the compressor, 5 microfarads for the fan), 40/5 MFD, 45/5 MFD, and 50/5 MFD. The replacement capacitor must match the original rating exactly. Installing a capacitor with the wrong MFD rating provides incorrect starting and running energy to the motor, which can cause immediate motor damage or premature motor failure. The voltage rating of the replacement can be equal to or higher than the original (a 440V capacitor can replace a 370V capacitor of the same MFD rating, but not vice versa).
How to Safely Inspect Your AC Capacitor
Capacitors store electrical charge even when the power is turned off. A charged capacitor can deliver a painful and potentially dangerous electrical shock. Do not touch the capacitor terminals, connecting wires, or the capacitor body until you are certain it has been properly discharged. If you are not trained in electrical work, limit your inspection to visual observation only and do not handle the capacitor.
If you want to visually inspect the capacitor yourself, follow these steps. First, turn off the AC at the thermostat. Second, turn off the power at the disconnect switch near the outdoor unit (a small gray or beige box on the wall near the condenser, with a pull-out handle or breaker switch). Third, wait at least 5 minutes. This allows any stored charge in the capacitor to partially dissipate, though it may not fully discharge on its own. Fourth, remove the screws from the outdoor unit's access panel (usually on the side of the unit). The capacitor is inside, typically mounted vertically near the top of the electrical compartment. It looks like a silver or black cylinder with wires attached to the top.
Look for the visual signs described above: bulging top, leaking fluid, charring, cracking, or melting. If you see any of these, the capacitor has failed and needs replacement. Replace the access panel, leave the disconnect off, and call a technician. If the capacitor looks normal visually, it may still have failed electrically. Only a capacitance test with a meter can confirm. Report what you observed to the technician when you call, as this information helps them arrive prepared with the correct replacement part.
Why Do AC Capacitors Fail?
Understanding why capacitors fail helps you take preventive steps and set realistic expectations for capacitor lifespan in your specific situation.
Heat Exposure
Heat is the primary killer of AC capacitors. The electrolytic fluid inside the capacitor degrades when exposed to sustained high temperatures. The outdoor AC unit generates significant heat during operation, and the ambient temperature in the electrical compartment of the outdoor unit can exceed 150 degrees Fahrenheit during peak summer operation in hot climates. Capacitors in Phoenix, Las Vegas, Houston, and Dallas (where the outdoor unit operates at maximum capacity for months at a time in 100 to 115 degree ambient temperatures) fail 30 to 50% sooner than capacitors in moderate climates like Seattle, Portland, or Minneapolis. Placing the outdoor unit in direct afternoon sun on the west side of the house increases the temperature inside the electrical compartment and accelerates capacitor degradation.
Power Surges and Voltage Irregularities
Power surges from lightning strikes, grid switching events, and brown-outs (voltage sags during peak demand) damage the thin dielectric layer inside the capacitor. A single significant surge can destroy a capacitor instantly, or repeated smaller surges can degrade it over time. Cities with frequent thunderstorms (Orlando, Tampa, Houston, Dallas, Kansas City) have higher capacitor failure rates for this reason. An HVAC surge protector ($100 to $300 installed on the outdoor unit) absorbs these surges before they reach the capacitor and other sensitive components. The $100 to $300 investment prevents $150 to $3,000 in component damage.
Age and Normal Wear
Capacitors have a finite lifespan regardless of conditions. The electrolytic fluid slowly evaporates or breaks down over thousands of operating hours. As the capacitance value decreases from its rated level, the motor receives less starting and running support, works harder, and eventually the capacitor value drops low enough that the motor cannot start or sustain operation. A technician can measure the actual capacitance and compare it to the rated value. A capacitor that tests below 90% of its rated MFD is considered weak and should be replaced proactively.
Overuse from Short Cycling
Every compressor startup demands a large burst of energy from the capacitor. In a normally operating system, this happens 3 to 5 times per hour. In a short cycling system, the compressor may start 10 to 20 times per hour, delivering 3 to 5 times more startup stress to the capacitor per hour than normal operation. This accelerated cycling degrades the capacitor faster and is one of the reasons that fixing short cycling promptly is important: the short cycling itself causes secondary component failures.
Manufacturing Defects
Some capacitors fail prematurely due to manufacturing quality issues. This is more common with low-cost aftermarket capacitors from unknown manufacturers. When replacing a capacitor, a quality OEM (Original Equipment Manufacturer) or reputable aftermarket brand (Titan Pro, Turbo, Packard) costs only $5 to $15 more than the cheapest available part but has significantly better reliability and a longer warranty.
Why a Blown Capacitor Can Destroy Your Compressor
This is the most important thing to understand about capacitor failure: a $150 to $400 capacitor problem that is ignored or delayed becomes a $1,500 to $3,000 compressor problem. Here is the mechanism.
When the capacitor fails or weakens, the compressor motor does not receive adequate starting energy. The motor windings energize (you hear the hum), generating a strong electromagnetic field, but the motor shaft cannot overcome the starting torque required to begin spinning. The windings are now drawing full locked-rotor amperage (LRA), which is 3 to 10 times the normal running amperage. This massive current flow generates significant heat in the motor windings. The thermal overload switch (a safety device built into the compressor) detects the excessive temperature and disconnects the circuit after 30 to 60 seconds. The motor cools down, the thermal overload resets, and the process repeats.
Each of these failed start attempts subjects the motor windings to extreme heat that degrades the electrical insulation coating on the copper wire. After dozens or hundreds of failed start attempts (which can occur over hours or days if the homeowner keeps the thermostat set to cool while the capacitor is failing), the insulation breaks down, the copper conductors short together, and the motor fails permanently. The compressor, which is a hermetically sealed unit containing both the motor and the refrigerant pump, cannot have its motor rewound or repaired. The entire compressor must be replaced at a cost of $1,500 to $3,000.
The lesson is simple: if your AC is humming but not starting, turn it off. Do not leave it running and "hope it works itself out." The system will not fix itself, and every humming-but-not-starting cycle damages the compressor. Turn the system off at the thermostat, call a technician, and leave it off until the repair is complete.
How Much Does AC Capacitor Replacement Cost?
| Service | Cost Range | Notes |
|---|---|---|
| Diagnostic/service call fee | $75 to $200 | Often credited toward repair |
| Dual run capacitor (part only) | $10 to $40 | 35/5, 40/5, 45/5, or 50/5 MFD |
| Start capacitor with relay (part only) | $30 to $75 | For systems with hard-start kits |
| Labor (standard hours) | $50 to $150 | 15 to 30 minutes of work |
| Total: standard replacement | $150 to $400 | During business hours |
| Total: emergency/after-hours | $250 to $600 | Evenings, weekends, holidays |
The capacitor itself is one of the cheapest parts in an AC system. A dual run capacitor costs $10 to $40 at wholesale. The majority of what you pay covers the technician's service call fee and labor time. Some companies charge a flat rate for capacitor replacement ($175 to $350 regardless of the capacitor type), while others charge a service call fee plus time and materials. Ask about the pricing structure when you call. For the complete pricing breakdown, see our AC capacitor replacement cost guide.
How Often Should You Replace Your Capacitor?
Most homeowners replace capacitors only when they fail, which is a reactive approach. A proactive approach involves checking the capacitor's health during the annual AC tune-up and replacing it when it tests below 90% of its rated value, even if it is still working. This proactive replacement costs the same $150 to $400 but happens during a scheduled maintenance visit rather than an emergency call, saving the $50 to $200 emergency surcharge and preventing the compressor damage that occurs when a capacitor fails during operation.
In hot climates (Phoenix, Las Vegas, Houston, Dallas), consider proactive replacement every 5 to 7 years. In moderate climates, every 8 to 10 years. If your system has a hard-start kit (an add-on that provides extra starting energy to the compressor, costing $100 to $200 installed), the run capacitor experiences less starting stress and may last longer. A hard-start kit is a worthwhile addition for any system over 7 years old, as it reduces the starting current demand and extends both capacitor and compressor life.
Can You Replace an AC Capacitor Yourself?
Technically, yes. Practically, it is not recommended for most homeowners due to the electrical shock risk. Here is an honest assessment of the DIY option.
The mechanical process is simple: turn off power, discharge the old capacitor, disconnect the wires (noting which wire goes to which terminal: C, HERM, and FAN), remove the old capacitor from its bracket, install the new one with the correct wiring, and restore power. The entire process takes 15 to 20 minutes for someone who has done it before.
The risk is in the discharge step. A run capacitor at 440V can store enough charge to deliver a significant electrical shock even hours after the power is turned off. The proper discharge procedure involves using an insulated screwdriver or a dedicated discharge resistor to bridge the terminals, allowing the stored charge to dissipate safely. If you skip this step or do it incorrectly, touching the terminals can deliver a shock that ranges from painful to dangerous depending on the capacitor voltage and your personal health conditions (particularly risky for anyone with a heart condition or pacemaker).
If you are comfortable with electrical work, have a multimeter to verify the power is off and the capacitor is discharged, and can correctly identify the replacement capacitor rating (matching MFD and equal or higher voltage), DIY replacement is feasible and saves $100 to $300 in labor. If you have any hesitation about working with stored electrical charge, the $150 to $400 professional cost is a reasonable price for safety. Our technician hourly rate guide explains typical labor pricing.
When Does a Blown Capacitor Signal a Bigger Problem?
A single capacitor failure on a system under 10 years old in a moderate climate is normal wear and tear. Replace it and move on. However, certain patterns indicate that the capacitor failure is a symptom of a deeper issue.
Repeated Capacitor Failures
If the capacitor has been replaced 2 or more times in 3 years, something is causing premature failure. Common culprits include a compressor that is drawing excessive amperage (the overloaded motor stresses the capacitor), voltage irregularities in the electrical supply (verify with a voltmeter; voltage should be within 10% of the rated value), undersized wiring to the outdoor unit, or a failed contactor that is delivering inconsistent power. A technician should investigate the root cause rather than simply replacing another capacitor.
Capacitor Failure on a System Over 12 Years Old
When a capacitor fails on an older system, the technician should check the compressor amp draw while they are there. If the compressor is drawing amps close to or above its rated value, the compressor is declining and may fail within 1 to 3 years regardless of the new capacitor. In this case, you may want to replace the capacitor now ($150 to $400) to get through the current season but begin planning for system replacement ($4,000 to $8,500) rather than investing in additional repairs on an aging system. Use our age decoder to check the system's manufacture date and the repair vs replace guide for the decision framework.
Capacitor Failure After a Storm or Power Event
If the capacitor failed immediately after a thunderstorm, power outage, or brownout, a power surge likely caused the failure. Install an HVAC surge protector ($100 to $300) when the capacitor is replaced to prevent repeat failures. The surge protector absorbs electrical spikes before they reach the capacitor, contactor, and compressor control board. In lightning-heavy markets like Orlando, Tampa, and Houston, a surge protector should be considered standard equipment rather than an optional add-on.
What Should You Tell the Technician When You Call?
Providing specific information when you call for service helps the technician arrive prepared with the likely replacement part and reduces diagnostic time.
Tell them what you observed: "My AC is humming from the outdoor unit but the compressor will not start" or "The fan on the outdoor unit is spinning slowly" or "The system starts, runs for a minute, and shuts off." Tell them what you checked: thermostat settings, breaker, filter. If you opened the access panel and saw a bulging capacitor, tell them. If you know the system brand and approximate age, mention it. If the failure happened after a storm or power outage, mention that too, because it tells the technician to also check for surge damage to other components.
If the technician diagnoses a capacitor failure and your system is under 10 years old, approve the repair. It is one of the most straightforward and cost-effective AC repairs. If the system is over 10 years old, ask the technician to also check the compressor amp draw and overall system condition while the access panel is open. This takes 5 minutes and gives you information about the compressor's health that helps you plan ahead.
Frequently Asked Questions About Blown AC Capacitors
The most common symptom is the AC hums from the outdoor unit but the compressor does not start. Other symptoms include the fan spinning slowly or not at all, the system shutting off shortly after starting, a clicking sound followed by nothing, a slight burning or chemical smell near the outdoor unit, and visibly bulging or leaking capacitor when inspected inside the access panel. The AC may also start normally but shut off after a few minutes if the capacitor is weak but not completely failed.
A failed capacitor often shows visible damage. The top of the capacitor, which is normally flat, may be bulging or domed outward. Oily residue or dried electrolyte fluid may be leaking from the base or connections. In severe cases, the capacitor may be charred, melted, or physically cracked. However, some failed capacitors look completely normal from the outside and can only be identified as failed through electrical testing with a capacitance meter.
You can visually inspect the capacitor for bulging, leaking, or damage by removing the outdoor unit access panel (turn off the power first at the disconnect switch). However, electrical testing requires a multimeter or capacitance meter, and the capacitor must be properly discharged first because it stores electrical charge even when the power is off. A charged capacitor can deliver a painful and potentially dangerous shock. If you are not trained in electrical work, leave the testing to a professional.
AC capacitor replacement costs $150 to $400 including parts and labor. The capacitor itself costs $10 to $40 for a standard model. The majority of the cost is the service call fee ($75 to $200) and the labor (15 to 30 minutes of technician time). Emergency and after-hours replacement adds $50 to $150 to the total. See our AC capacitor replacement cost guide for detailed pricing by capacitor type.
Yes. A failing capacitor that provides insufficient starting power forces the compressor motor to strain during every startup attempt. The motor draws excessive amperage trying to start with inadequate capacitor support, which overheats the motor windings. Repeated stressed startups degrade the winding insulation over time, eventually causing the compressor to fail. A $150 to $400 capacitor replacement prevents a $1,500 to $3,000 compressor failure.
AC capacitors last 5 to 15 years depending on climate, usage, and electrical conditions. In hot climates like Phoenix, Las Vegas, Houston, and Dallas where the AC runs 7 to 9 months per year, capacitors last 3 to 7 years. In moderate climates with 3 to 5 months of cooling, 8 to 12 years is typical. Power surges, voltage irregularities, and excessive heat from direct sun exposure on the outdoor unit all shorten capacitor life.
Technically possible but not recommended for most homeowners. The primary safety risk is electrical shock: capacitors store electrical charge even when the power is off and can deliver a dangerous shock if not properly discharged before handling. The discharge process requires a specific tool (an insulated screwdriver or discharge resistor) and knowledge of the correct procedure. Additionally, installing the wrong capacitor rating can damage the compressor motor. Professional replacement costs $150 to $400 and eliminates the risk.
A start capacitor provides a large burst of energy to start the compressor motor from a standstill. It is active for only 1 to 2 seconds during startup and then disconnects. A run capacitor provides continuous energy to keep the compressor and fan motors running at the correct speed and power factor. Run capacitors fail more often because they are active continuously during operation. Many systems use a dual run capacitor, a single unit that serves both the compressor and the fan motor, with three terminals instead of two.
