High coolant temperature

You should wash the rad/condensor coil.It's located at top front of the unit behind the grill.Leave the reefer running and spray water onto the coil for 10 minutes or so.You can just use a garden hose or a pressure washer but don't put the nozzle to close to the coil if using pressure as you could flatten the fins on it.
Scroll through the controller and check the water temperature readings before and after wash and see if there is a large difference. Good luck

 

I suppose the the problem could be the temperature sensor or wiring. I had a similar problem in my truck when periodically the coolant tem p guage would just all of the sudden go to 200-250 degrees and immediately come back down to 180. I discovered the insulation on one of the wires to temp sensor had worn to exposed wire and was touching the engine when hitting bumps.