Sometimes the logic circuit in the board gets into a loop for some reason. Usually this is indicated by a one-flash code but there have been a couple times where this procedure cleared a no-flash fault as well.

If you have been unsuccessful at resetting the fault code from the thermostat:

Remove the access cover to the interior electronics. On the circuit board next to the thermostat connector your will see three pin socket labeled CN-18 (with or without pins; see photos below). The pins may be covered with silicone. You are going to jump the center pin and the pin closest to the thermostat connector with a paper clip, screwdriver tip or the like. (See the photo below showing you which pins you'll be jumping.) You can just poke the paper clip right through the silicone to contact the pins.Note: The heater must be powered on while doing this procedure, so be careful not to touch anything other than the two pins. Start by doing the code reset, verify you have no flashes (before you attempt to start the heater) and then try to start the heater so you get your one-flash code. Jump the two pins described above for a few seconds and the light should (in theory) go out. Put the cover back on the heater and try to start. If all goes as planned the one-flash code should now be completely out of the logic circuit's memory and the heater should run (assuming the things that actually caused the fault have been remedied) as normal.  

OR:

If there is no change after the CN18 procedure, try testing the Propex thermostat.

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