When a garage door opener starts to smoke 99% of the time it has a burnt out capacitor. The capacitor is kind of like a big fuse and it is what starts and stops the motor and it is about 3” long and tubular in shape it is full of chemicals and paper and when it loses its seal the paper inside starts to burn from the electrical wires heating up causing the capacitor to smoke and sometimes while it is smoking it might make a whistling sound.

A capacitor is by far one of the inexpensive parts to replace on an opener unit. Every door technician should have one on his truck at all times because it is a very common problem. If a technician tells you your unit needs to be replaced because its smoking then ask him to just replace the capacitor and see if that solves the problem. If a new capacitor does not solve the malfunction then a new opener unit may be needed.