How to Check Contactors on an Air Conditioner

Turn off the power to the external air conditioner compressor by locating the fuse above it inside a covered metal box. Flip the fuse down, or pull it out of the wall completely, depending on the model.

Examine the bottom of the compressor to locate the screws that hold the cover in place. Remove each screw using a screwdriver or a power drill and set them safely aside. Grasp the cover and lift it up to expose the internal parts of the compressor.

Identify the contactor, which looks similar to a light switch, except it has no front lever and a coil rests on the rear of it. The electrical wires also connect to the contactor on the top or bottom.

Look closely at the contactor to identify any black char material, which is carbon buildup, or dented or stippled surfaces. The appearance of any of these indicates the contactor is failing and it requires immediate replacement.

Examine the metal components on the contactor to determine if they are melted together instead of moving freely. If this is the case, the contactor has already failed and requires immediate replacement. This condition is also characterized by the inability to shut the system off at the thermostat.

Restore power to the unit by inserting or flipping the fuse. Turn on a multimeter tool and set it to resistance. Insert one of the metal prongs into the left side of the rear coil on the contactor and the other metal prong on the right side of the coil. Look at the readout on the voltmeter to ensure it is 20 ohms, which is normal. If the reading falls above or below this, the contactor is failing.