When the fan in the outdoor unit stops turning, the air conditioner can no longer shed heat. Cooling drops, the outdoor unit gets hot and the compressor may trip on protection. The two usual suspects are the run capacitor and the fan motor, and the symptoms often tell them apart. If the fan has stopped, switch the AC off at the controls until it has been checked, because running the compressor without the fan can damage it.
Safety first. Switch off the power at the breaker or isolator before you open the outdoor unit. A capacitor can hold a charge after the power is off, so do not touch its terminals. Leave electrical testing to a qualified technician.
Sign 1: The Fan Hums but Does Not Turn
This is the classic capacitor symptom. With the power off, turn the fan blade gently by hand. A fan that spins freely by hand but will not start by itself points to a weak or failed run capacitor.
Sign 2: The Fan Feels Stiff or Gritty
If the blades are hard to turn, or you feel grinding as you spin them, the bearings may be worn or seized. That points to the motor, not the capacitor, and a new capacitor will not fix it.
Sign 3: The Capacitor Looks Damaged
A swollen top, oil leaking from the case or burn marks mean the capacitor has failed. A capacitor can also fail without any visible sign, so a tidy-looking one is not proof that it is good.
Sign 4: The Fan Runs Slowly or the Motor Is Hot
A fan that turns but much more slowly than before can be a weakening capacitor or worn bearings. A motor that is very hot to the touch, smells burnt or trips the breaker points to a winding fault in the motor itself.
Inverter Units Are Different
Some inverter air conditioners use a DC fan motor, which is driven by the control board and has no run capacitor. If the fan will not run on a DC model, the fault may be the board, a connector or the motor, so have the board output tested before you replace the motor.
Buying the Right Part
For a capacitor, match the microfarad (µF) rating and the voltage printed on the old one, and for a dual capacitor match both values. For a motor, match the voltage, power, speed, direction of rotation, shaft size and mounting, and check whether it is an AC or a DC type. Browse our AC capacitors and outdoor unit fan motors, or send us a photo of the old part's label.
If the compressor has been running hot, read our guide to the warning signs of a failing AC compressor, and keep up a regular service schedule so problems like this are caught early.