I can see this thread is old, but non-the-less, it gave me some insight into my recent E05 sighting on my Mastertemp 400 heater. Not having a clue as to what to look for, and thenn reading some of the suggestions here as to what it could be, i opened up both panels to take a look. Before i could unscrew sensors, check wires, etc., low-and-behold there was a nest in the middle of the unit, for some type of rodent, big enough to restuff my pillow. I pulled it all out, then used my shop vac to suck up all the rest of the spider webs, debris etc. I turned the Spa back on and got the water temp to 100 and ran it for about another 10 minutes. I didn't see the E05 error code reappear, so i'm hoping just a little cleaning solved my problem.