The burner fires with a stable flame, but the flame goes out after a while and a flame fault is indicated. This happens frequently enough that I’m able to observe the heater when the flame goes out.

  1. Press the reset button, observe the flame, listen to the burner components (air compressor is the loudest), and wait for the flame to go out. Observe the delay (if any) between the time the flame goes out and the time the air compressor stops running. The difference you’re looking for is between absolutely no delay (0 seconds), or a delay of anything over 1 second.
  2. An alternative to this test is to have someone in front of the heater and someone at the heater exhaust. Press the reset button, observe the flame, observe the exhaust, and wait for the flame to go out.