Carrier is one of the most common AC brands in residential and light commercial. Here is a systematic approach to diagnosing no-cool calls.
Outdoor Unit Not Running At All
### Check Power Verify 240V at the disconnect. Check the breaker. Check for a blown fuse in the disconnect box.
### Check Contactor The contactor is the relay that sends power to the compressor and fan motor. Check for 24V across the coil terminals when the thermostat calls for cooling. If you have 24V but the contactor is not pulling in, replace it. If the contacts are pitted or burned, replace it. Cost: $15-35.
### Check Capacitor The dual run capacitor is the single most common AC failure. Test with a capacitance meter. If either the HERM or FAN side reads more than 5% below rating, replace it. Cost: $10-35.
Outdoor Unit Runs But No Cooling
### Check Refrigerant Charge Connect gauges. Low suction and high superheat indicate a low charge (leak). High suction and low superheat indicate an overcharge or metering device issue.
### Check Condenser Coil A dirty condenser coil causes high head pressure and poor cooling. High discharge pressure with the fan running normally points to a dirty coil. Wash with a coil cleaner and garden hose.
### Check Compressor If the compressor hums but does not start, check the start components. If it runs but does not pump (suction and discharge pressures equalize quickly), the compressor has internal valve failure.
Indoor Unit Issues
### Check Evaporator Coil A frozen evaporator coil indicates low airflow (dirty filter, closed registers) or low refrigerant charge. Let it thaw completely before restarting.
### Check Blower Motor If the blower is not running, check the blower motor capacitor first (cheapest fix). Then check the motor itself and the control board relay.
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