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Here’s one of two limit switches in my furnace. This was pulled out while I was attempting to determine why my furnace isn’t working properly. As it’s nice and clean this didn’t seem to be the reason for the problem.
What I’ve observed is as follows:
1) Pilot igniter sparks for ~20 seconds.
2) Pilot lights.
3) ~20 seconds later main gas valve opens.
4) Main burner ignites.
5) Main burner extinguishes, but pilot stays lit.
6) 4-5 repeat three times before blower starts.
After removing this limit switch, looking it over, cleaning its contacts and surrounding area, then reassembling it #5 didn’t occur and the blower started like normal. I’ll listen carefully for a while and see if the problem reoccurs or if more thought needs to be given to the issue.
I’d generally blame the flame sensor, but there isn’t one on my furnace. I’m concerned that this problem may be caused by the controller board dying (capacitor plague?) or some other passive electronic bit slowly failing, leading to intermittent issues. Replacing controllers or gas valves is not cheap, and I’m wary of doing so without knowing that it’ll fix the problem.