so let me try get this clear, the thermostat calls for heat, the furnace does its purge cycle then lights, runs for a bit and goes cold.
1, is it actualy lighting? are you getting hot air then it shuts off? or is it trying to light then failing?
I had the issue where it would light, sometimes, but when it shutoff and went to relight the next time the thermostat called for heat it wouldn't light.. then eventualy it wouldn't light at all. this ended up being bugs in the exhaust path and even inside the main burner. once I took it all apart and cleaned it it worked perfect untill I lost a board. with this it wouln't light at all. would go through the purge cycle but just wouldnt light.
Yes- it's lighting and will run for maybe 5 minutes max at hot air- sometimes it shuts off and then comes on with cold air- othertimes the hot air just transitions to cold air. He's out tearing it completely apart now- we have access to a SF35 that we could get parts from if needed- from what we are reading the cores are identical not sure about the thermacouplers and modules though.......
