What does it mean when my AC compressor won't start but the fan runs

The outdoor fan is separate from the compressor and runs off its own motor. A compressor that won't turn on while the fan spins indicates an electrical or refrigerant problem, not a mechanical one. This is usually fixable without full system replacement.

Most common cause: failed capacitor

The capacitor is a cylindrical component next to the compressor that gives it the electrical kick to start. When a capacitor fails, the compressor hums but won't turn. A new capacitor costs under fifty dollars in parts but requires a technician to replace safely because capacitors retain charge. Do not attempt this yourself. This is the single most common reason a compressor won't start.

Other possible causes

A failed contactor, which is an electrical switch that closes to power the compressor, creates the same symptom. Low refrigerant also triggers safety pressure switches that prevent compressor startup. These conditions all require professional diagnosis with a multimeter and pressure gauge. Turn off your system and call a technician. Running the compressor without refrigerant or with a failed capacitor causes permanent damage. This is not a situation where you should experiment or delay.