Troubleshooting Central Air Conditioning

If you're troubleshooting central air conditioning that's running but seems to be cooling poorly, our Air Conditioning and Refrigeration System Evaluation Manual has a cycle diagram with information about what pressures and temperatures to look for, and guidance on how to evaluate abnormal readings.

If you're troubleshooting central air conditioning problems on a chilled water system that's running but seems to be cooling poorly, the same Air Conditioning and Refrigeration System Evaluation Manual has cycle diagrams, a log sheet, and guidance on what to look for when troubleshooting a chiller.

If you're troubleshooting central air conditioning equipment that's manufactured by Trane, our page about troubleshooting Trane air conditioning controls has some useful information about Trane's electronic controls.

If you're working on a system and it seems totally dead, the first thing you have to do is check the thermostat.

Make sure there is control power to it, that it is turned on to cool, and that the contacts are closed to the cooling and fan circuits.

If there is no control power, find out why and correct it.

If it wasn't turned on, turn it on and see if the unit runs.

If it's turned on but the contacts don't close, the thermostat has failed.

If it's a programmable thermostat, check the programming and verify that it's programmed correctly.

If the thermostat is working correctly, next check the air handler.

If the blower isn't running, check the breakers and disconnect, and verify that you have the correct voltage at the air handler.

If you don't, find out why, and correct the problem.

If the air handler still doesn't run:

Check the relay, contactor, or motor starter for the motor.

Is there control voltage?

If there's control voltage but the relay, contactor, or starter isn't energized, it's failed.

If the relay, contactor, or starter is energized, verify that the load side voltage is correct.

If the load side voltage is correct and the motor isn't running, you'll need to verify that you have the correct voltage at the motor terminals.

A wire might be broken between the relay, contactor or starter, and the motor.

If you have good voltage at the motor terminals, turn off power, and check the connections and the winding resistances.

You probably have a failed motor.

If you're troubleshooting central air conditioning with the air handlers running but there's no cooling, you need to check the condensing unit.

The following tips on this page are a quick guide to troubleshooting central air conditioning condenser problems.

For a more detailed explanation of the procedure, you might find our Central Air Conditioning Condenser Problem page helpful.

Check the circuit breaker and disconnect, and verify that you have the correct voltage at the unit.

If not, find out why and correct the problem.

If the unit still won't run:

Verify that there is refrigerant in the system.

Check for the "cool" signal from the thermostat.

Check for a failed control component.

Check for an open safety.

Check for a loose connection or broken wire.

Check for a failed compressor or condenser fan motor.

By the time you've completed all of the above checks, you'll have found the problem.

Time for a diet soda.

I hope this page has helped, and please, feel free to contact us with any specific HVAC questions you might have, including questions about air conditioning on Guam, or refrigeration on Guam.