Compressor trouble diagnosed with a meter, not a hunch

The compressor pumps refrigerant through the system and does most of the heavy work of moving heat out of your home. When it stops, cooling stops, and because a compressor is one of the most expensive parts in an air conditioner, a failure there feels like the worst possible news. In practice, a good share of compressors that appear dead are not. The motor is fine, but something that feeds it power or helps it start has failed.

A proper diagnosis looks at the whole chain: the run capacitor, the contactor, the wiring and terminals, the internal overload, the motor windings and the refrigerant circuit. Each one can stop a compressor from running, and most of them are far cheaper to fix than the compressor itself.

When the compressor genuinely has failed, the conversation changes. We explain whether a compressor replacement makes sense for your system's age and condition, what the alternatives cost, and the steps required to protect the new compressor if you go ahead.

A compressor motor needs a burst of help to overcome the pressure in the system when it starts. The run capacitor provides a phase shift that helps the motor start and run efficiently. As a capacitor ages, its capacity drops, and eventually the compressor cannot start reliably. Replacing a failed capacitor is one of the most common and least expensive fixes in residential cooling. In some cases a hard-start kit, which adds a start capacitor and a relay, gives an older compressor the extra torque it needs to start against pressure.

A hard-start kit is useful, but it is not a cure for a compressor with damaged windings or worn internal parts. If testing shows a winding shorted to ground or a mechanically locked compressor, adding start assistance will not fix it and can mask the problem briefly. That is why the windings are tested before any start components are sold. When a compressor burns out electrically, the refrigerant and oil can become acidic, and any replacement must include an acid test, a filter drier and proper cleanup to protect the new compressor.

The word compressor gets used loosely for any outdoor unit problem, but most calls about a compressor that will not run turn out to be something around it. The compressor depends on a run capacitor to start and stay efficient, a contactor to deliver power when the thermostat calls, an overload protector to shut it down when it overheats, and in some cases a start capacitor or hard start kit to help it get moving. Any of these can fail and make a healthy compressor look dead.

Testing begins with power. The technician confirms voltage is reaching the unit and passing through the contactor when the thermostat calls for cooling. Capacitors are measured against their rated values, because a capacitor can look normal and still be well below specification. If the compressor is hot, it may be sitting on its internal overload, which resets only after it cools down.

Next come the windings. Measuring resistance between the common, start and run terminals shows whether the windings are intact, open or shorted, and a test from each terminal to the compressor body shows whether a winding has shorted to ground. A grounded or open winding usually means the compressor has failed. A compressor with good windings that hums without starting may be mechanically stuck or simply need starting help.

If the compressor runs, refrigerant pressures and amperage draw tell the rest of the story. High amperage with normal pressures can point to internal wear. Low suction and high discharge pressures may indicate a restriction. These readings let you decide with confidence whether to repair the component, replace the compressor or replace the condenser.