Mastertemp 400 cycling


New member
Aug 2, 2014
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?


TFP Expert
Mar 2, 2011
Possibly mice or nesting material in the blower or flameholder.

Check everything thoroughly.

See this reference.

Check the stack flue temperature, gas pressure and the flame current.

Check the incoming voltage and make sure that the correct voltage selector plug is installed.

Do NOT install the 120 volt plug if you have 240 volts coming in. If you do, you will seriously damage the heater
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