AC running but not cooling: step-by-step HVAC troubleshooting guide
AC running but not cooling? Field troubleshooting for thermostat settings, airflow, evaporator and condenser coils, refrigerant circuit and electrical components.
An AC running but not cooling may look like a low-charge problem, but many faults create the same symptom: incorrect thermostat mode, low airflow, a frozen evaporator, blocked condenser, stopped fan, unpowered compressor or an abnormal refrigerant circuit. Good troubleshooting starts by separating what is actually operating from what only sounds like it is.
Both Carrier and Trane list thermostat settings, filters, coils, airflow and refrigerant among the main checks when an air conditioner runs without cooling. Review the official guides from Carrier and Trane.
1. Confirm the cooling demand
Verify COOL mode, setpoint, schedule and fan setting. FAN ON can keep the blower running between cooling cycles and make the system appear to blow warm air. Then confirm the Y call at the thermostat and control voltage at the equipment according to the manufacturer's diagram.
2. Identify what is truly running
- Is the indoor blower moving normal airflow?
- Is the outdoor fan operating in the correct direction?
- Is the compressor running, or only the fan?
- Does the contactor receive its call and close correctly?
- Is a fault code, protection or time delay active?
Noise from the outdoor unit does not prove that the compressor is pumping. Measure voltage and current with the appropriate safety procedure.
3. Verify airflow before judging pressures
Inspect the filter, return, registers, evaporator, blower wheel, motor speed and ductwork. Low airflow reduces evaporator load, lowers suction pressure and may create ice. Adding refrigerant in that condition makes the diagnosis worse. Measure air temperature split and, when appropriate, total external static pressure.
4. Read the evaporator and condenser
A frozen evaporator must be fully thawed before measurements are interpreted. Outdoors, check cleanliness, airflow, ambient temperature, recirculation and fan condition. Poor heat rejection can create high head pressure, high current and reduced capacity.
5. Measure the refrigerant circuit
Identify the refrigerant and metering device first. Record suction and discharge pressures, saturation temperatures, suction-line temperature and liquid-line temperature. Calculate superheat and subcooling. One pressure alone cannot prove low charge: airflow, restriction, load, outdoor temperature and compressor performance all matter.
6. Check the electrical side
If the compressor or a motor will not start, verify supply voltage, voltage drop, contactor, capacitor, overload, windings and connections. An out-of-tolerance capacitor or low voltage can leave a fan running while the compressor fails to start normally. Disconnect and lock out power before resistance or capacitance tests.
7. Know when to stop the system
Shut equipment down for a heavily frozen evaporator, repeated breaker trip, overheated compressor, stalled motor or pressures outside safe limits. Running equipment that cannot transfer heat can turn a localized fault into an expensive failure.
Field report checklist
- Thermostat mode, setpoint and calls.
- Return/supply temperatures and temperature split.
- Filter, evaporator, condenser and fan condition.
- LP, HP, saturation, superheat and subcooling.
- Voltage, current and electrical component condition.
- Proven cause, correction and final readings.
FrigoTechPro brings P/T charts, calculators, nameplate scanning and an HVAC-R AI assistant into the same field record. Continue with the frozen evaporator guide and the low suction/high head guide.