1. Thermostat in Heat Mode (or Set Wrong)
Sounds obvious, but it's our #1 false alarm — and we don't charge for the diagnostic visit when it's just thermostat configuration. Smart thermostats can switch modes during firmware updates, after power outages, or when family members reconfigure them.
Quick check: confirm the thermostat is set to "Cool" not "Heat" or "Off." Verify the temperature setpoint is below the current room temperature. For heat pumps, ensure "Auxiliary" or "Emergency Heat" is not enabled. Check the schedule isn't putting you in an Eco mode you don't want during the day.
2. Refrigerant Leak / Low Charge
The most common actual mechanical cause. AC systems require precise refrigerant charge to cool effectively. As the charge drops below spec (from a slow leak), the system can no longer absorb heat from indoor air — leading to warm air at the vents.
Diagnosis: technician puts manifold gauges on the system and measures suction/discharge pressures. Low pressure on both sides indicates undercharge. Then we use an electronic leak detector to find where the refrigerant is escaping.
Repair cost: $400-$1,200 typical, depending on leak location. EPA Section 608 rules require us to repair the leak before recharging — we can't legally just "top off" a leaking system.
3. Frozen Evaporator Coil
When the indoor coil ices over, it can't absorb heat from the air — and the air passing through warms slightly instead of cooling. Causes: low refrigerant (see #2), restricted airflow (clogged filter, blocked return), or running the AC when outdoor temperature is below 60°F.
Diagnosis: visual inspection of the evaporator coil and refrigerant lines. Ice or frost is obvious. Turn off the system to thaw (typically 1-3 hours), then identify the underlying cause.
Repair cost: $250-$650 if the cause is filter or charge issue. Higher if the coil itself is damaged from repeated freeze cycles.
4. Failed Compressor
The compressor is the heart of the AC system. When it fails — typically from age, electrical issues, or running with low refrigerant — the system can't pressurize refrigerant and therefore can't move heat. The fan may still run, blowing room-temperature air.
Diagnosis: ammeter reading at the compressor (zero amps with system calling for cooling = compressor failure or stuck contactor). Resistance check across compressor windings. Sometimes the compressor is locked rotor — drawing locked-rotor amps without spinning.
Repair cost: $1,800-$3,500 for compressor replacement. On systems 10+ years old with R-22 or older refrigerant, full system replacement often makes more economic sense.
5. Condenser Fan Motor Failure
If the outdoor unit's fan stops spinning, the compressor can't reject heat to outside air. The system pressures spike, the high-pressure switch trips, and cooling stops. Some systems then continue running the indoor blower (now without effective cooling) and you get warm air at vents.
Diagnosis: check the outdoor unit. If the compressor is running but the fan is not, it's the fan motor or capacitor. Voltage and capacitance testing identify which.
Repair cost: $400-$900 for fan motor; $180-$350 for capacitor.
6. Failed Capacitor
The dual-run capacitor stores energy to start the compressor and condenser fan motor. When it fails (usually visibly bulged or leaking), the compressor and/or fan can't start, leading to no cooling at the registers.
Diagnosis: visual inspection (bulging top is obvious). Microfarad measurement with a capacitance meter — must be within ±6% of nameplate spec.
Repair cost: $180-$420 for capacitor replacement. We carry the four most common sizes on every truck.
7. Stuck or Failed Contactor
The contactor is an electromechanical switch that energizes the outdoor unit when the thermostat calls for cooling. Pitted or stuck contacts can prevent the compressor from getting power, causing the system to "run" (indoor blower active) without actual cooling happening.
Diagnosis: voltage check at the contactor — should see 24V on the coil and 240V at the line side. If line-side voltage isn't reaching the load side when the coil is energized, the contactor needs replacement.
Repair cost: $200-$380.
8. Ductwork Issues
Sometimes the AC is fine, but the cool air isn't reaching the rooms. Causes: collapsed flex duct, disconnected duct in attic or crawlspace, major duct leakage, restricted return air. Air at the air handler may be cold, but air at distant registers is warm.
Diagnosis: inspect ductwork visually where accessible (attic, crawlspace). Measure supply temperature at the air handler vs at distant registers — large delta indicates duct loss.
Repair cost: $300-$3,500 depending on extent. Aeroseal duct sealing ($1,800-$2,800) is highly effective for leakage; physical duct repair varies.
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