Are you a technician doing some air conditioning troubleshooting on a totally dead unit? Is it a nice hot summer day? Did you just finish a service call 20 minutes ago where you were sweating buckets wriggling around in somebody's steam-kettle attic? If you're done with your diet soda, grab your tools, crank up the can-do attitude, and let's go troubleshoot this air conditioner.

This unit's acting dead, so check the breaker, check the disconnect and fuses, and check to make sure you have correct power supplied to the unit. If the breaker is tripped and/or a fuse is blown, check the unit for grounds or shorts.

Disconnect the leads from the compressor terminals and check for grounded, shorted, or open windings. Check the evaporator and condenser fan motors for grounded or shorted windings. Check the control and power circuits for shorts or grounds.

If your air conditioning troubleshooting call is for a unit that has good supply voltage but is otherwise dead:

If your supply power checks out OK, make sure the thermostat is turned on, that there is control power to it, and verify that it actually works. If there's control power to the thermostat and it works, make sure the wires to the evap fan relay and condensing unit aren't broken at the thermostat, or somewhere between the thermostat and the units.

When the evaporator fan won't run:

Check for power at the evap fan relay. If it gets power but the contacts don't close, it has failed. If it gets power and the contacts close, but the fan doesn't run, check for voltage on the load side of the relay, check the leads to the fan, test the fan windings and the capacitor, and check to see if something is stuck in the blower and stopping it from turning.

When the condenser won't run:

Verify the cooling control signal from the thermostat. Check for refrigerant pressure. If the refrigerant has leaked, the low-pressure safety will open and the unit will not run.

Check the safeties. If the oil safety switch has tripped, check the oil level. It will also trip if the contactor closes but the compressor doesn't run for some reason, so check for open compressor windings, verify that the compressor terminal connections and contactor connections are tight, and verify that there is actually good voltage at the compressor when the contactor pulls in.

If the high-pressure safety is open, check your pressures. If head pressure has dropped below the cut in point of the safety, and it is an automatic reset type, the safety has failed. If the safety is a manual reset type, push the button and see if it will reset.

Check the voltage monitor if there is one. If your voltage is within range of the setting, the safety contacts should be closed. If they're not, the monitor has failed.

If there is a delay timer, check across the control terminals. If the delay timer contacts never close, it has failed.

If the safety contacts are closed, you should have control power at your contactor. If there is no control power at the contactor coil, check for a broken wire somewhere. If you have control power at the contactor coil but the contactor doesn't pull in, the contactor has failed.

If you're on an air conditioning troubleshooting call where the contactor pulls in but the compressor doesn't run, check for open compressor windings, verify that the compressor terminal connections and contactor connections are tight, and verify that there is actually good voltage at the compressor when the contactor pulls in.

If it's a single-phase compressor, check the start relay and the capacitors. The best way to test the start relay and capacitors is to replace them with new parts.

If the condenser fan doesn't run, check the relay, motor windings and capacitor, and fan blade, the same way as with the evaporator fan.

Be thorough as you trace down wiring and look for failed components, and always follow safety precautions during your air conditioning troubleshooting jobs.

By the time you reach this point of air conditioning troubleshooting, you will have found the failed component and repaired or replaced it, and the unit will be ready to run. Good job!

A technician attaches his gauges and checks pressures and temperatures. (Photo courtesy of York.)


When air conditioning troubleshooting on a unit that runs but doesn't seem to be cooling efficiently:

Check the evaporator and condenser fans. Verify the motors are the correct horsepower, and that they rotate in the proper direction and at the correct rpm. Verify that the fan and blower blades are clean, that they're the correct size and pitch, and that they are turning in the right direction.

Verify that the evaporator and condenser coils are clean and airflow is not blocked. Verify that the air filter is clean.

Verify that the condenser is getting normal temperature outside air, and that another unit isn't blowing hot air into it. Verify that the evaporator supply and return ducting is not leaking; losing cold air or picking up warm outside air.

At this point in this air conditioning troubleshooting job, attach your gauges and check your pressures and temperatures.

With the unit off and pressures equalized, verify that the system contains the correct refrigerant. You can do this by taking the temperature of the evaporator coil. It should match the pressure/temperature indication on your low side gauge or pressure/temperature chart.

Run the unit and check operating pressures. When the space has cooled down and is about 5 degrees above design temperature, look for the following pressures and temperatures.

  • Suction pressure should be in the range of 35 to 40 degrees below return air.

  • Discharge pressure should be in the range of 20 to 35 degrees above ambient air.

  • Superheat should be 20 to 30 degrees at the compressor.

  • Subcooling should be 10 to 15 degrees at the outlet of the receiver or condenser.

  • Air temperature rise through the condenser should be 20 to 30 degrees.

  • Air temperature drop through the evaporator should be 15 to 20 degrees.

    If all of your operating characteristics fall within these ranges, the unit is running well.

    At this stage of air conditioning troubleshooting, if there is still a concern about the unit not cooling properly, you will need to do a capacity check.

    Measure the airflow in cfm through the evaporator. Measure the wet bulb temperatures of the air entering and leaving the evaporator.

    Using a psychrometric chart or an enthalpy conversion table, convert the wet bulb temperatures to enthalpy values, and calculate the difference between the two values. Multiply the difference in enthalpy values times 4.5, which is a constant used in this calculation, and then multiply that product times the cfm. This will tell you how many Btu of heat the evaporator is absorbing from the air flowing through it.

    Yes, air conditioning troubleshooting often involves some mathematics.

    If the difference between design capacity and running capacity is minor, it can be adjusted by adjusting evaporator blower speed. If the unit is running at or near design capacity but still not handling the load of the space being cooled, the unit is simply too small.

    Mike Taitano is a technician with 20 years of experience troubleshooting, repairing, and maintaining air conditioning and refrigeration equipment. He also operates For more information, visit

    Publication date: 07/10/2006