I can't figure out why my Mastertemp 400 would cycle on and off and then usually after 3 cycles the service light would light (and no errors on the control board). I typically do the general maintenance of the heater...thermal regulator, thermistor etc. I had a service technician come and tell me I needed an entire new unit....said the heat exchange was bad due to poor chemistry. I knew this was not the case. I took apart the unit and inspected the heat exchange....and as I expected no leaking, no corrosion on the heat exchange. The one thing I did note was the top insulation pad was in pieces....I'm assuming a rodent (unsure if that would have an effect on the system cycling...but order new ones. Waiting for them to come in). The other thing I noticed was the grill on my air orifice was chewed...unsure how far the rodent could travel in the combustion blower. I haven't seen any chewed wires either. Could the issue be within the Flameholder?