AC fan runs but compressor won't start
AC fan runs but compressor won't start: what it means
When the outdoor fan spins but the AC compressor is not turning on, the system can’t move heat out of your home. You’ll feel warm air at the registers, the thermostat will never hit setpoint, and energy use may climb. This symptom points to an electrical, refrigerant, or safety-control problem. While a few simple checks are safe for homeowners, many causes involve live voltage, pressure controls, or refrigerant circuits that require licensed HVAC expertise. Insurance carriers and equipment warranties commonly require service by a qualified technician and prohibit bypassing safety devices.
Quick checks you can do safely
- Thermostat settings: Confirm COOL mode, setpoint at least 3–5°F below room temp, and fan on AUTO. Weak batteries can create thermostat issues—replace them if uncertain.
- Wait 5 minutes: Many systems have short-cycle protection. After a call for cooling, the compressor may intentionally delay.
- Circuit breaker: Check the outdoor condenser breaker and the indoor air handler breaker. Reset once if tripped. If it trips again, stop and call a pro.
- Outdoor disconnect: Ensure the pull-out or breaker next to the condenser is fully seated. Do not open live components.
- Air filter: A severely clogged filter can overheat the system and trigger safeties. Replace if dirty.
- Outdoor airflow: Clear leaves, plastic bags, or debris around the condenser. Maintain at least 2–3 feet of clearance and rinse off a dirty condenser coil from the outside in, power off first.
- Condensate overflow/float switch: A full drain pan can open a float switch and stop cooling. If water is present, you may have a clogged drain that needs service.
Common causes when the fan runs but the compressor does not
- Bad capacitor: A failed start/run capacitor is the classic reason a fan runs but the compressor won’t start. Signs include a swollen capacitor or a humming outdoor unit. Capacitors store energy even with power off; testing and replacement should be done by a professional.
- AC contactor issues: Pitted or burnt contacts on the AC contactor can pass enough power to run the fan motor but starve the compressor. Replacing a contactor involves live-voltage work and lockout/tagout safety.
- High pressure switch trip: A dirty condenser coil, blocked airflow, or a failed fan speed can raise head pressure and trip the high-pressure switch. Do not bypass a high pressure switch. Correct the root cause, clean the coil, and have a technician reset if needed.
- Low pressure switch trip or low refrigerant: A leak or restriction can open the low-pressure switch to protect the compressor. Running the system in this state can cause damage. EPA rules require a certified technician for refrigerant work.
- Dirty condenser coil: Even if the fan runs, a dirty condenser coil can cause overheating and prevent the compressor from staying on. Regular cleaning under proper safety procedures helps, but heavy fouling needs a deep professional clean.
- Compressor thermal overload: A compressor can overheat due to poor airflow, high ambient temps, or inadequate voltage, and open its internal protector. After cooling down, it may retry. Repeated trips signal underlying issues.
- Start relay or hard-start needs: Some compressors need a proper start-assist kit. A failed relay or missing hard-start support can leave the compressor humming without engaging.
- Thermostat or control board faults: Miswiring, a failed control board, or sensor errors can energize the fan circuit while leaving the compressor circuit off. Smart thermostats and communicating systems add complexity that requires skilled diagnostics.
- Voltage problems and wiring: Loose lugs in the disconnect, a failing breaker, burnt spade terminals, or a weak transformer can keep the compressor from engaging even though the fan runs.
- Float switch behavior: A partially triggered float switch might interrupt the Y call intermittently, confusing diagnostics. Clearing the drain and verifying proper slope is key.
- Heat pump mode quirks: On heat pumps, the reversing valve, defrost control, or outdoor sensor faults can lead to unusual behavior. If your unit is a heat pump, mention that when seeking service.
Helpful sound cues
- Humming without start: Often a bad capacitor, tight compressor, or low voltage.
- Rapid clicking: Contactor chattering from low voltage or a control issue.
- Silence while fan spins: The compressor circuit may not be receiving a call or is open on a safety switch.
Why DIY fixes are risky
Capacitors can deliver a dangerous shock. High-pressure refrigerant circuits and spinning components are hazardous. Bypassing a pressure switch or forcing the contactor can damage the compressor. Many insurance policies and most manufacturer warranties require work by licensed technicians, documented according to code. Attempted DIY or unlicensed repair can void coverage.
Preventive steps to reduce repeat failures
- Seasonal maintenance: Clean the dirty condenser coil, confirm refrigerant charge, tighten electrical connections, and test capacitors and the AC contactor under load.
- Drain reliability: Clear and treat condensate lines; verify float switch operation.
- Airflow: Keep filters clean and maintain 2–3 feet of clearance around the outdoor unit.
- Thermostat and surge protection: Stable control voltage and surge protection reduce nuisance failures.
Commercial and residential considerations
In commercial and residential buildings, especially large or complex projects, a single “fan runs but compressor not starting” event can hint at broader issues—undersized electrical service, airflow imbalances, or control sequencing errors. For property managers, documenting cause, corrective action, and test results is essential for compliance and risk management.
About #1 AC Guys
#1 AC Guys is a family-owned team of engineers with 80+ years in engineering across four generations. Our mission is healthy air at home for a healthy life, and we work on commercial and residential systems, including large and complex projects. Our engineers have factory training at Fujitsu (Japan), Mitsubishi Elektrik (Thailand), Midea (China), Gree (China), and Hier (China). We serve Los Angeles County and nearby areas, including Orange County, Ventura County, and Western Riverside County. Cities we frequently serve include Los Angeles, Long Beach, Pasadena, Glendale, Burbank, Santa Monica, Torrance, Irvine, Anaheim, Santa Ana, Costa Mesa, Huntington Beach, Garden Grove, Fullerton, Oxnard, Ventura, Thousand Oaks, Corona, and Ontario.
If your AC fan runs but the compressor won’t start, remember that modern systems are protected by multiple safeties for a reason. The safest and most cost-effective path is proper diagnostics, repairs to code, and complete documentation that align with insurance and warranty requirements.
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