AC not cooling: guided troubleshooting
Warm air from the vents with the thermostat calling for cooling. Answer each question from what you see on site.
Step 1
Is the thermostat display on, set to cool, and set below room temperature?
See every step in this flow
- Is the thermostat display on, set to cool, and set below room temperature?
- Yes: Is the indoor blower running?
- No, it's blank or dead: Start at the thermostat and the 24 V supply
- Is the indoor blower running? Feel for air at a supply register or listen at the air handler.
- Yes: What is the outdoor unit doing?
- No: Indoor unit isn't running
- What is the outdoor unit doing?
- Nothing at all: Outdoor unit has no power or no call
- Humming or buzzing, not running: Humming but not starting
- Fan runs, compressor doesn't: Compressor isn't starting
- Compressor runs, fan doesn't: Condenser fan isn't running
- Fan and compressor both running: Is there ice or frost on the indoor coil or the large suction line?
- Starts, then shuts off: Short cycling
- Is there ice or frost on the indoor coil or the large suction line?
- Yes: Frozen coil
- No: Is airflow right? Clean filter, clean indoor and outdoor coils, registers open.
- Is airflow right? Clean filter, clean indoor and outdoor coils, registers open. A dirty outdoor coil or weak indoor airflow will throw off every pressure and temperature reading that follows.
- Something is dirty or blocked: Fix airflow first
- Airflow checks out: Measure superheat and subcooling. What do you have?
- Measure superheat and subcooling. What do you have? Compare against the unit's target superheat (fixed orifice) or subcooling (TXV).
- High superheat, low subcooling: Low charge
- High superheat, high subcooling: Restriction
- Low superheat, high subcooling: Likely overcharged
- Low superheat, low subcooling: Metering device overfeeding, or compressor not pumping
- Both near target: The refrigerant circuit looks right
Where each path ends
Start at the thermostat and the 24 V supply
A blank or dead thermostat usually means no 24 V: dead batteries, a tripped breaker or service switch, an open door switch, a blown low-voltage fuse on the board, or a tripped condensate float switch.
Indoor unit isn't running
Check for a cooling call (24 V at Y and G), the low-voltage fuse, the float switch and the door switch. If the blower has a call and power but doesn't run, test the motor and its capacitor or module.
Outdoor unit has no power or no call
Check the disconnect and breaker for line voltage, then for 24 V at the contactor coil. No 24 V points back to the thermostat wire, a safety switch or the indoor board.
Humming but not starting
Most often a failed run capacitor or a pitted contactor. A compressor that hums and trips on overload with a good capacitor needs further testing.
Compressor isn't starting
Check the capacitor's compressor side, voltage at the compressor terminals, and whether the compressor is hot and out on its internal overload.
Condenser fan isn't running
Shut the unit off before head pressure climbs. Check the fan side of the capacitor, then the motor.
Short cycling
Find out what is shutting it off: a pressure switch, the compressor overload, the thermostat or a control fault.
Frozen coil
Shut the compressor off and thaw the coil with the blower before taking any readings. Then look for low airflow first and low charge second.
Fix airflow first
Replace the filter, clean the coils and re-check. Don't adjust charge on a system with an airflow problem.
Low charge
The system is short of refrigerant, which means a leak. Find and fix it before adding refrigerant.
Restriction
Refrigerant is backing up in the condenser and starving the evaporator. Look for a temperature drop across the filter drier, a kinked line, or a metering device that isn't feeding.
Likely overcharged
Confirm indoor airflow once more, since low airflow also drives superheat down. If airflow is right, recover refrigerant to the manufacturer's target.
Metering device overfeeding, or compressor not pumping
Compare pressures. High suction with low head pressure points at the compressor (or a leaking reversing valve on a heat pump). Otherwise look at a TXV stuck open or a wrong-size piston.
The refrigerant circuit looks right
Look outside the refrigerant circuit: duct leakage, sizing, a high heat load, or humidity. Work through the full checklist.