How to Test an AC Compressor

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Updated September 16, 2022

Even the best air conditioners can sometimes break down, and a faulty compressor is a concern for any AC owner. This is important, because you don’t want to just dispose of your air conditioner, in case you’re wondering how to scrap an AC unit. Because caring for your cooling unit can often be confusing, we’ll show you how to test an air conditioner compressor in this article. And for more information on proper air conditioning unit care, you read our resource on how to turn off an air conditioner.


  • The compressor circulates the refrigerant through the condenser coils that cool the inflowing air.
  • To test your compressor, you’ll need a specialty tool called a multimeter.
  • If your tests read consistently over 30 ohms, you will probably have to replace the compressor.

What is an Air Conditioner Compressor?

The compressor is perhaps the most critical part of an AC unit. It’s responsible for circulating the refrigerant through the system’s condenser coils so that it effectively cools the air. This is different than the dry setting on an air conditioner. Think of it as the heart of the machine, which is why you should do all you can to keep it in top condition. Another critical part of AC repair is the capacitor. For this, we have a great resource on how to test an AC capacitor.

Insider Tip

A wave or diode symbol should represent the continuity setting on a multimeter.

Testing Your AC Compressor

STEP 1 Power Down your AC Unit

Go to the fuse box with the circuit breaker panel that powers your AC unit. Switch off the power supply.


Remove the Access PanelUnscrew everything that fastens the access panel. Remove the panel so that the electronics and wires are showing.

STEP 3 Check for Damage

  1. Inspect the wires, compressor, and capacitor for visible damage. Check for any ruptures, burn marks, frayed wires, or a blown fuse.
  2. The compressor is a cylindrical black or silver tube with three wires connected to it.
  3. Specifically, check the compressor’s terminals (the metal parts that the wires connect to) for burn marks.
  4. If no damage is detected, you’ll have to inspect it with a multimeter.

STEP 4 Locate the C, R, and S Terminals

  1. Get out your multimeter and turn it to the “continuity” setting.
  2. Place the red node on the C terminal and the black node on the S terminal.
  3. Look at the ohm reading to check for proper voltage. It should read significantly below 30 ohms.
  4. Leave the red node on C, and move the black node to R.
  5. Check the ohm reading. Again, if it is above thirty, you likely have a bad compressor.
  6. Finally, put the red node on the R and the black node on the S. This number should also read under 30.
  7. If the numbers read consistently above thirty, you need to replace your compressor


You can purchase and replace an AC compressor yourself, but this is often expensive and better handled by an HVAC professional.


What are some signs of a bad AC compressor?

Common signs of a bad air conditioning compressor are loud noises coming from the machine and a lack of hot airflow from the outside unit.

Is it necessary to have a compressor in good working order?

Not only does the compressor regulate the cooling process, but a damaged compressor can cause a major leak of refrigerant, which can be physically harmful.

Are all AC compressors built the same?

There are different types of AC compressors, particularly between your home and vehicle. However, they operate very similarly.

STAT: The acceptable ohm range is usually labeled on the capacitor. If the capacitor tells you “50 ±5%,” the acceptable range is 47.5 to 52.5 F. (source)

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