You're standing over the outdoor unit on a hot day: the thing is humming, maybe the compressor is even running, but the big top fan sits dead still. Three parts produce this exact scene, and they separate with one careful observation — before any panels come off.
The (careful) stick test
With the unit calling for cooling and humming, reach through the top grille with a long stick, never fingers, and give a fan blade a gentle push. If the fan catches and runs: the motor works but can't start itself — the fan side of the run capacitor is dead. That spin-up is your diagnosis, not your fix: a motor starting on a dead capacitor strains and overheats every cycle, and it will take the motor with it within weeks. If it doesn't catch: the motor itself has likely seized or burned (a hot, ozone-smelling motor housing is the confession), or nothing's reaching it — see the contactor.
The contactor: the relay everyone forgets
The contactor is the switch that sends power to the whole outdoor unit when the thermostat calls. Pitted or burnt contacts feed the fan and compressor weakly or not at all; a contactor welded shut does the opposite — the unit runs even with the thermostat off. If the unit is totally silent (no hum), the contactor, its 24V control signal, or the breaker is the story rather than the fan itself.
Why a still fan is urgent
The fan exists to carry heat away from the compressor. Without it, head pressure soars and the compressor cooks until its internal overload trips — run it that way repeatedly and you're auditioning for the most expensive repair in air conditioning. If the fan is down, shut cooling off at the thermostat until it's fixed; the house being warm for a day is cheaper than a compressor.
The repairs, sized up
Capacitor: inexpensive, minutes, in the van. Contactor: similarly modest. Fan motor: a standard mid-range repair — matched by horsepower, RPM and rotation, with a new correctly-sized capacitor installed alongside as a set (a new motor on an old capacitor is a warranty claim waiting to happen). All three are same-visit fixes with a written quote after the meter confirms which one earned the blame — and in a heat wave, no-cool calls jump our queue.