How do I know if my AC capacitor has failed?

The most common symptoms of capacitor failure: (1) The condenser unit hums but the compressor does not start — the compressor attempts to start, draws high current, and trips on the internal overload protector; (2) The condenser fan does not spin when you can see or hear the compressor running; (3) The condenser unit starts briefly and then shuts off, sometimes tripping the breaker; (4) The system runs but cools less effectively than it should — a degraded capacitor that has not fully failed yet may allow the compressor and fan to run but at reduced efficiency. Only a capacitor microfarad measurement confirms the diagnosis; the symptoms are consistent with several other failure modes.

Can I replace the capacitor myself?

Technically, a capacitor swap is a simple mechanical task once the unit is safely de-energized and the capacitor is fully discharged. The reason we do not recommend homeowners attempt it is the discharge step — an AC capacitor charged to 440 volts stores enough energy to cause cardiac arrest. Disconnecting power at the breaker does not discharge the capacitor; it must be discharged through a resistive load before it is safe to touch. Additionally, matching the replacement capacitor to the correct microfarad and voltage specification requires knowing the nameplate rating and confirming the replacement value with a meter. The $89 diagnostic and capacitor replacement cost is the cost of doing this correctly and safely.

How long should a capacitor last?

In standard operating conditions, residential dual-run capacitors typically last 10–20 years. In high-ambient locations — south- and west-facing condenser locations in SunCrest, Corner Canyon, and South Mountain where condenser cabinet temperatures regularly exceed the capacitor's rated operating temperature during peak summer — effective service life can be 5–10 years. We track capacitor replacement history in your service file and factor prior replacement dates into our replacement-threshold recommendations during tune-ups.

Why did my capacitor fail again after just two years?

Three most likely causes: (1) The replacement capacitor was downrated — either the microfarad value was slightly below the original spec, the voltage rating was insufficient for the installation's utility voltage, or the quality tier of the replacement part was lower than the original. (2) The condenser is installed in a high-ambient location where thermal degradation accelerates significantly compared to a shaded installation. (3) An underlying electrical issue — low or high utility voltage, single-phasing from a failing contactor, or a failing motor drawing above FLA — is causing the replacement capacitor to work harder than normal. The last scenario requires diagnosing the root cause, not repeatedly replacing the capacitor.

What is the difference between the capacitor and the contactor?

A capacitor stores and releases electrical energy to assist motor starting and running. A contactor is an electrically-operated switch that connects the high-voltage supply (240V) to the compressor and condenser fan when the low-voltage thermostat signal calls for cooling. Both components are in the condenser unit's electrical compartment and both can fail in ways that prevent the system from starting. A failed capacitor typically allows the contactor to close (you hear the click) but prevents the compressor from starting. A failed contactor may not close at all, may weld closed (system runs continuously), or may create intermittent contact. We test both components during every repair visit where a no-start symptom is reported.

AC Capacitor Replacement in Draper, Utah

A failed capacitor is the most common cause of an air conditioner that hums but does not start, a condenser fan that spins when you give it a push but won’t start on its own, or a compressor that trips the breaker immediately after startup. It is also one of the cheapest AC repairs — a dual-run capacitor, properly diagnosed and replaced with a microfarad-verified part, typically runs $185–$285 parts and labor. The catch is that a failed capacitor produces symptoms that look identical to compressor failure to a technician who does not pull out a capacitor tester and measure the component before quoting the replacement.

We carry an extensive cross-reference stock of dual-run and start capacitors on our service trucks. Most capacitor replacements in our service area are completed on the initial diagnostic visit without a return trip for parts.

What a Capacitor Does

An AC system’s single-phase condenser uses capacitors to start and run the compressor and condenser fan motor. Single-phase induction motors cannot generate starting torque from a single-phase power supply alone — the capacitor creates a phase-shifted current in the start winding that produces the rotating magnetic field needed to initiate motor rotation.

A dual-run capacitor performs two functions in one component: it stores and releases energy to assist the compressor motor start sequence, and it maintains a phase-shifted current in the condenser fan motor’s run winding to improve fan motor efficiency during continuous operation. The dual-run capacitor is labeled with two microfarad ratings and one voltage rating — for example, “45+5 MFD 370/440V” means the compressor side is 45 microfarads and the fan side is 5 microfarads, rated for 370 or 440 volt AC service.

A start capacitor is a separate component sometimes found on older or higher-horsepower compressors that need additional starting torque. Start capacitors are in circuit only during the start sequence (typically 300–500 milliseconds) and are switched out of the circuit by a potential relay or current relay once the motor reaches operating speed. Start capacitors are not as common in modern residential equipment as in commercial single-phase or older residential units.

How Capacitors Fail

Thermal Degradation

Electrolytic capacitors degrade thermally over time — the dielectric fluid inside the capacitor evaporates slowly through the seal, reducing the capacitor’s stored charge capacity (its microfarad rating) as the fluid depletes. The rate of degradation follows the Arrhenius relationship: roughly double the degradation rate for every 10°C increase in operating temperature above the rated temperature.

This has specific implications for south Salt Lake Valley installations. A dual-run capacitor installed inside an outdoor condenser cabinet has a rated maximum operating temperature that assumes standard ambient air conditions. On a 96°F afternoon at a SunCrest or Corner Canyon condenser location on a south-facing wall — where measured ambient at the condenser can reach 106–110°F — the capacitor inside the condenser cabinet may be operating at 80–85°C rather than its rated 70°C. At 80°C versus 70°C, the Arrhenius-projected service life is reduced by approximately 50%. A capacitor with a manufacturer-rated 60,000-hour service life at 70°C may have an effective service life of 30,000 hours at high-ambient SunCrest and Corner Canyon installations.

At approximately 1,200–1,500 hours of annual runtime for a residential condenser in the south Salt Lake Valley cooling season, 30,000 hours represents roughly 20 years of service at rated temperature but only 10 years at elevated ambient. The capacitor that was installed when your SunCrest home was built in 2014 and has been running on a south-facing wall ever since may have been at or near its effective end-of-life capacity for the past two or three seasons.

Voltage Transients

Power quality events — brief voltage surges from nearby lightning strikes, utility switching events on Dominion Energy’s distribution network, and the startup transients of large motors on the same circuit (well pumps, pool pumps, large air compressors) — accelerate capacitor degradation by stressing the dielectric film with voltage spikes above the capacitor’s rated peak voltage. The 370V/440V dual voltage rating on modern residential capacitors provides some margin; capacitors from older equipment rated for 370V only have less headroom. Draper’s position at the base of the Wasatch Range makes it a relatively active lightning area during summer monsoon season, with the July–August afternoon thunderstorm pattern regularly producing nearby ground strikes in the SunCrest, Traverse Ridge, and Corner Canyon areas.

Age and Cycling Stress

Each compressor start event puts the capacitor through a charge-discharge cycle. A single-stage compressor in a south Salt Lake Valley home during peak summer cycles approximately 6–10 times per hour on a 95°F day. At 10 hours of peak-temperature daily runtime during July and August, that is 600–1,000 start cycles per month during peak season. Over a 10-year system life, the cumulative charge-discharge cycles add up to approximately 72,000–120,000 cycles. Capacitor manufacturers do not publish cycle life ratings for residential HVAC capacitors the way they do for industrial applications, but the thermal degradation model typically remains the binding constraint before cycling stress becomes the primary failure mode.

How We Diagnose Capacitor Failure

Capacitor diagnosis requires a capacitor tester that can measure actual microfarad output with the capacitor out of circuit and fully discharged. A multimeter set to capacitance works; a dedicated capacitor tester is faster and more accurate for the microfarad ranges used in HVAC capacitors (typically 2.5–80 MFD). We use a Fluke 117 or equivalent capacitance-capable meter, not a visual inspection.

Important safety note: An AC capacitor charged to 370 or 440 volts retains a potentially lethal stored charge even after power to the unit is disconnected. Before measuring a capacitor, a trained technician discharges it through a resistive load (a 20,000Ω, 5-watt discharge resistor is the standard tool) and confirms discharge with a voltmeter. Do not attempt to remove or measure an AC capacitor without discharging it first. This is one of the HVAC tasks that should not be attempted by homeowners regardless of the apparent simplicity of the component swap.

Our diagnostic sequence for a suspected capacitor failure:

Confirm the symptom: compressor does not start (hums, trips on overload), condenser fan does not start (spins freely by hand when power is off), or neither component starts reliably.
Check contactor pull-in and confirm voltage is reaching the compressor and fan terminals (eliminates contactor failure, disconnect issues, or low-voltage control problems from the diagnosis).
Disconnect power and discharge the capacitor per the procedure above.
Measure microfarad output on both the compressor and fan sides of the dual-run capacitor (or on the separate start and run capacitors if equipped).
Compare measured microfarad values against nameplate ratings. Our replacement threshold: below 90% of nameplate on either side gets flagged; below 80% of nameplate on either side gets recommended for immediate replacement.
If the capacitor passes and the symptom persists, proceed to compressor winding resistance testing and contactor face inspection.

Replacement Parts

We replace capacitors with parts that meet or exceed the original equipment’s microfarad and voltage rating. For dual-run capacitors, the replacement must match the original within the manufacturer’s tolerance on each side (typically ±6% on the compressor side, ±6% on the fan side) and must have an equal or higher voltage rating. We do not downrate capacitors to clear a lower-cost part through the repair — a 370V-only capacitor should not replace a 440V unit in a location where utility voltage runs high, and a capacitor with a lower microfarad value on the compressor side than the original spec will produce the same symptom as the failed capacitor within one to three seasons.

Our trucks carry a range of dual-run capacitor values covering the most common condenser models in our service area: 35+5, 40+5, 45+5, 50+5, 55+5, 60+5, and 40+7.5, 45+7.5, 50+7.5 microfarad ratings, in both 370V and 440V versions. For less common values (e.g., older Trane or American Standard equipment with non-standard ratings), we source from Watsco or Johnstone Supply same-day or next-morning in most cases.

Hard-Start Kits

A hard-start kit (also called a start assist kit or SPP, start potential relay + potential start capacitor) is an add-on component that supplements the dual-run capacitor during the compressor start sequence. Hard-start kits are used when:

A compressor is showing elevated start amperage draw that does not exceed the LRA rating but suggests increasing starting difficulty (common on 10–15-year-old compressors)
A compressor is installed in a location with documented low utility voltage during peak demand periods (some Riverton and Bluffdale distribution feeders see voltage drop during July and August afternoon peaks)
A single-phase compressor is starting against a partially loaded refrigerant system (unequalized pressures at startup) because the refrigerant migration check valve or crankcase heater is not functioning
The previous capacitor has failed and the compressor’s start winding has accumulated some thermal stress from the failed-capacitor start attempts

A hard-start kit does not fix a failed compressor and does not substitute for a correct capacitor. It is an appropriate addition to a capacitor replacement when the diagnostic findings support its use. We include the recommendation in the written repair quote with the specific reason for the recommendation, not as a default upsell on every capacitor replacement.

Frequently Asked Questions

How do I know if my AC capacitor has failed?: The most common symptoms of capacitor failure: (1) The condenser unit hums but the compressor does not start — the compressor attempts to start, draws high current, and trips on the internal overload protector; (2) The condenser fan does not spin when you can see or hear the compressor running; (3) The condenser unit starts briefly and then shuts off, sometimes tripping the breaker; (4) The system runs but cools less effectively than it should — a degraded capacitor that has not fully failed yet may allow the compressor and fan to run but at reduced efficiency, manifesting as longer-than-normal run times to reach setpoint. Only a capacitor microfarad measurement confirms the diagnosis; the symptoms are consistent with several other failure modes.
Can I replace the capacitor myself?: Technically, a capacitor swap is a simple mechanical task once the unit is safely de-energized and the capacitor is fully discharged. The reason we do not recommend homeowners attempt it is the discharge step — an AC capacitor charged to 440 volts stores enough energy to cause cardiac arrest. Disconnecting power at the breaker does not discharge the capacitor; it must be discharged through a resistive load before it is safe to touch. Additionally, matching the replacement capacitor to the correct microfarad and voltage specification requires knowing the nameplate rating (not always legible on an aged capacitor) and confirming the replacement value with a meter. The $89 diagnostic and capacitor replacement cost is the cost of doing this correctly and safely.
How long should a capacitor last?: In standard operating conditions (ambient temperatures at or near the manufacturer’s rated operating temperature), residential dual-run capacitors typically last 10–20 years. In high-ambient locations — south- and west-facing condenser locations in SunCrest, Corner Canyon, and South Mountain where condenser cabinet temperatures regularly exceed the capacitor’s rated operating temperature during peak summer — effective service life can be 5–10 years. We track capacitor replacement history in your service file and factor prior replacement dates into our replacement-threshold recommendations during tune-ups.
Why did my capacitor fail again after just two years?: Three most likely causes: (1) The replacement capacitor was downrated — either the microfarad value was slightly below the original spec, the voltage rating was insufficient for the installation’s utility voltage, or the quality tier of the replacement part was lower than the original. (2) The condenser is installed in a high-ambient location (south or west facing in SunCrest or Corner Canyon) where thermal degradation accelerates significantly compared to a shaded or north-facing installation. (3) An underlying electrical issue — low or high utility voltage, single-phasing from a failing contactor, or a failing motor drawing above FLA — is causing the replacement capacitor to work harder than a normal capacitor and failing it prematurely. The last scenario requires diagnosing the root cause, not repeatedly replacing the capacitor.
What is the difference between the capacitor and the contactor?: A capacitor stores and releases electrical energy to assist motor starting and running. A contactor is an electrically-operated switch that connects the high-voltage supply (240V) to the compressor and condenser fan when the low-voltage thermostat signal calls for cooling. Both components are in the condenser unit’s electrical compartment and both can fail in ways that prevent the system from starting. A failed capacitor typically allows the contactor to close (you hear the click) but prevents the compressor from starting (you hear a hum or silence). A failed contactor may not close at all (no click, no start), may weld closed (system runs continuously regardless of thermostat), or may create intermittent contact that causes erratic cycling. We test both components during every repair visit where a no-start symptom is reported.

Contact Draper Heating & Air Conditioning

Most capacitor replacements are completed on the same visit as the diagnostic in Draper, Sandy, Bluffdale, Riverton, South Jordan, and Herriman. Call our 24/7 dispatch line for emergency no-cool situations or schedule a standard diagnostic visit.

Emergency Line (24/7): (385) 336-1837
Address: 12244 Business Park Dr #155, Draper, UT 84020
Email: info@draperheatingairconditioning.xyz
Utah DOPL HVAC Contractor License: #11487612-5501
EPA Section 608 Universal: #608U-2011-318472

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AC Capacitor Replacement Draper | Draper Heating & Air