I have an old (20 yrs) Lennox gas-fired forced air furnace model #G12QSE-127-7. It has a Honeywell retrofit ignition model S8660K. I've started having the following problem when the thermostat calls for heat:
- pilot valve opens and spark ignites pilot
- spark will continue intermittently for 10-15 secs
- after 30-60 secs the main valve fails to open and furnace shuts down.
This does not happen every time, but has become more and more frequent. I already replaced the flame sensor, no change. I called in a repairman who said it was the control box. I bought a replacement 53L90 kit and installed the controls, still no change.
The repairman showed me how to manually "trip" the main valve open by crossing a wire over the main valve and the pilot valve terminals. That causes the main valve to open and the furnace ignites immediately. Great, but not a long-term solution.
Any thoughts on what else to troubleshoot here? I've reached the limit of my knowledge on furnace electrical.
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