Step 1: Check the motor's smell. If there is a burning smell, replace the motor immediately.
Step 2: Check the motor's input voltage. Ensure wires are not damaged and the proper power supply is connected.
Step 3: Listen for loud vibration or squeaking noises. Such noises may indicate damaged or worn-out bearings. If possible, lubricate the bearings, otherwise replace the motor completely.
Step 4: Check for overheating. Use compressed air to rid the motor of debris, allow to cool, and restart.
Step 5: AC Motors that make an effort to start but fail, may be a sign of a bad starting capacitor. Check for any signs of leaking oil, and replace the capacitor if this is the case.
Step 6: Ensure the application the motor is rotating is not locked up. Other mechanical components may be the cause of failure or poor performance. Do this by disconnecting the mechanism and try running the motor by itself.