Reasons an Air Conditioner Compressor Will Not Start

There are many things that can cause a compressor to stop working.

Often the compressor is good, but another component has failed, which stops it from working properly.

Below is a list of some common issues to look for when troubleshooting a unit that will not start.

5 Reasons an Air Conditioner Compressor Will Not Start

1. Bad Capacitor


A bad Start or Run capacitor will not start a compressor and will need to be replaced if bad.

The Start capacitor gives extra amps when needed to help the compressor start, while a Run capacitor allows it to run smoothly with no heavy up and down spikes.

Most air conditioner units will have a Start capacitor labeled as HERM, but designs can vary.

Testing the capacitor is easy enough with an HVAC multi meter. They are also low-cost parts to replace.

2. Condensing Fan Motor Failure


A bad condensing fan motor will stop a compressor from working. This can also be tied to a bad capacitor since Start Run capacitors are used to start the condensing fan motor.

When the Fan motor goes bad, the compressor will kick on for a short while then stop when the Head Pressure builds up.

Since the heat is not dissipated away from the unit, the amps and heat will build up, making the compressor overload and trip off.

If a unit is left on, the compressor will cool down and kick back on, going into a bad cycle of running for a short while, overheating and kicking off.

Often an Air Conditioner becomes loud when this happens.

This could potentially damage the compressor since the pressures can get high and pull a lot of amps.

3. Contactors or Relay


The contactors or relay controls the main high 220-volts coming into a unit.

It uses a low-voltage contactors relay to control the high voltage, which can go bad and not allow any voltage through.

A low-voltage thermostat is used to trip a contactor which allows 220 volts into the unit powering it on.

Common problems include pitting or the relay coil winding itself going bad.

Read Here for How To Replace Contactors or Relays.

4. Thermostat


A bad thermostat or thermostat-wiring will cause a unit not to start.

Be sure to check the thermostat is set on cool and at a low enough temperature to turn on the unit.

HVAC techs often use a jumper wire to trip the unit where the thermostat wires go into the system.

For example, most air conditioners use the yellow wire to cool and red as the 24DC hot wire.

When the yellow and red wire are jumpered, the unit should pull in on the contactor powering on the unit.

5. Electrical Short

If the winding on the compressor goes bad, it will cause the unit to ground out.

The breaker will throw when this happens. It can happen for many reasons, such as a small bug getting into the unit and the compressor wiring or simply age.

6. Float Controls

All HVAC units need to drain water when it is humid. If the drain pans or hoses become backed up, float controls can turn a unit off.

Summary
The above are a few of the main problems an HVAC air conditioner or heat pump unit can have.

Each component will need to be tested to find the problem and be replaced.

It can, of course, simply have a mechanical failure and stop working. If you have run into problems, let us know in the comments below.