Most common causes
- Thermostat set wrong. If the thermostat is on Heat, Auto fan only, or the setpoint is above room temperature, the system will move air without cooling it.
- Clogged air filter. A dirty filter chokes airflow across the indoor coil. The coil can get too cold and freeze, and then very little cool air reaches the rooms.
- Outdoor unit has no power. The indoor fan can run while the outdoor condenser sits dead because its breaker tripped or the disconnect switch is off. With no compressor running, the air just circulates at room temperature.
- Dirty outdoor coil. Dust, leaves, and cottonwood fluff on the condenser fins trap heat. The system cannot shed heat outdoors, so it cannot cool indoors.
- Failed capacitor or contactor. These electrical parts start the compressor and outdoor fan. When one fails, the outdoor unit may hum or stay silent while the inside blows warm air.
- Refrigerant leak. Refrigerant does not get used up. If the charge is low, there is a leak somewhere in the coils or lines, and it has to be found and repaired before the system is recharged.
- Leaky or disconnected ductwork. A duct that came loose in the attic can dump cold air into hot attic space and pull hot attic air into the system.
What you can safely check
- Set the thermostat to Cool and at least 3 degrees below room temperature. Replace thermostat batteries if the screen is blank or dim.
- Pull the air filter and hold it up to light. If you cannot see light through it, replace it.
- Check your main electrical panel for a tripped AC or condenser breaker. Reset it once. If it trips again, leave it off and call.
- Make sure the outdoor disconnect switch near the condenser is in the On position.
- Clear leaves, weeds, and debris at least 2 feet around the outdoor unit and gently rinse the fins with a garden hose from the outside in, with the power off at the disconnect.
- Go outside while the system runs and listen. If the fan is not spinning or the unit only hums, turn the AC off to protect the compressor.
Call a technician if
- The breaker trips again after you reset it
- The outdoor unit hums, clicks, or stays silent while the thermostat calls for cooling
- You see ice on the copper lines or hear hissing near the coils
- Filter and thermostat are fine but the air from the vents is still warm after 15 minutes
- You notice oily spots on refrigerant fittings or a burning smell
What affects the repair cost
The cost depends on what failed. A thermostat or filter issue costs little or nothing. Electrical parts like a capacitor or contactor are usually a moderate repair. A refrigerant leak costs more because the technician has to locate the leak, repair it, pull a vacuum, and recharge, and the price depends on where the leak is and the refrigerant type your system uses. Compressor failure on an older system often leads to a repair versus replacement conversation.
