MasterTemp 400 Issue


Mar 16, 2021
I have a MastertTemp 400 that is intermittent. When the heat command is given, the blower will start and the heater will fire. Sometimes it will stay lit, other times it will only burn for about 5 to 10s and turn off. It will immediately try again to lite after the fan purge. Sometimes it will stay lit after the retry and other times it won't. After a few tries it gives up and says service heater.

I took the side panel off and checked th diagnostic LEDs while the service heater light was on. This is where I am stumped and what triggered my post. None of the LEDs indicating what was at fault are lit. Does this indicate a circuit board issue?

Thanks in advance for any help. If it is circuit board related, hopefully I can hold out for that new rs485 board that is pending release!


TFP Expert
Mar 2, 2011
This shows the inlet tap to the gas valve and below is a gas pressure test kit that comes with a brass fitting that will screw into the tap port (thread is 1/8" npt).

Turn off the gas valve outside of the heater before removing the test port plug.

When the test adapter is screwed in, connect the pressure gauge and then open the gas valve outside of the heater.

Check the pressure and make sure that no gas is leaking.

Start the heater and see what happens to the pressure like in the video.

After the test, turn off the gas valve and reinstall the test port plug and make sure that it does not leak.

Note: Don't do anything that you are not sure that you can do safely.

If you are not sure about what to do, contact a service that can do the job correctly.

Mastertemp gas valve inlet tap.jpgYellow Jacket test kit.jpg
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