Unplug the air conditioner or set the electrical panel breaker to the “off” position before beginning work. Air conditioning units have an internal part called a capacitor, which stores electricity even when the unit is unplugged. If you meet the capacitor, it can deliver a life-threatening shock. Only attempt this troubleshooting if you are confident that you recognize the capacitor and can avoid it. The capacitor is usually no longer than 4 inches and is shiny. It comes in a variety of shapes, but is often round or oval. It is found in various places depending upon the specific air conditioning unit.
Open the air conditioner’s case by unscrewing or unlatching it. Look for the circuit board is similar to what you would find on a computer. Find where the main power comes into the AC system. Between these two points is a small box with wires running into it from the main power supply and wires running to the circuit board. This box is the transformer.
Visually inspect the box’s outside. Replace transformers that appear deeply discolored, burned or are leaking liquids. Wires that are crimped in a door or chewed by animals may cause a short in the transformer. Open the box, which is often closed with either screws or a simple clip. Look inside for evidence of burning or excessive heat. Pull the fuse out and carefully examine it. If the fuse is bad, a replacement may correct the problem. The fuse may be on the circuit board instead of in the transformer.
Turn the power back on and check the connections with the multimeter. The connections leading from the transformer to the circuit board are the secondary outputs and should read between 24 and 28 volts. If your digital multimeter reads “OL,” it means you have an open line and the transformer is bad. If an analog multimeter shows a very small voltage, it means the same thing. Turn the power back off as soon as you finish the reading.