I was called upon to try and fix a GFCI in a bathroom that occasionally trips. There is a 12/2 20 amp feeder coming into a three gang plastic box. In the box are the GFCI, a single pole switch to a fan, and a dimmer switch to a lighted medicine cabinet. Occasionally, the GFCI trips after you turn the overhead fan off (not on, but off.) All three units are pigtailed from the feed and the GFCI is wired to the "Line" side. I changed both switches out but not the GFCI (I didn't have one with me.) I also disconnected the hotwire to the dimmer. I've reached the point in the process of elimination where I think I will have to pull the fan unit but would prefer not to if someone has a better idea.

I'd replace the GFCI with a quality one. The devices attached suggest possible nuscense tripping.

If it continues to trip move the fan outside of the GFCI circuit.