There really arent that many parts to a Z4000 We currently have 24 of them at the club and have only had one with a part problem All the other problems were minor fixes The first thing you should check is the circuit protector on the side that doesnt work If you see any white showing just push it back in and thats your fix. If that isnt it there are 7 Torques screws on the bottom that can be removed Look at the gear mesh on the handle that you are having a problem with That little doo dad on the gear actually does the changes in your voltage There is a line that needs to be lined up with the other gear for proper adjustment After that look for simple problems Like a wire touching, a fan not turning etc There is a way of swapping the inputs to the handles to check the circuits but I bet thats not it