I have an older York system, Model # P2MPD20L08001A, installed in 1994.
It has an intermittent issue as follows: after it reaches the end of the heating cycle and it's time to shut down, the flame cuts out and the blower fan should stop after about 50 seconds or a little longer, but the blower fan will continue to run until I unplug the unit. If I don’t unplug the unit, the blower fan will run all day and the thermostat will no longer control the system.
After I unplug the unit I can plug it back in after a few moments and the blower fan will stay off until it's ready for the next heating cycle. Once I've "rebooted" it by unplugging it, it will work fine for the next 10-50 cycles of heat, then the problem returns. It happened 3 or 4 times last winter, but this year it's happening almost every day, so I need to fix it.
As I said, the problem is intermittent, and seems to happen more overnight (when it's cold in the basement) than during the day. I'm in Los Angeles, so "cold" is a relative term. Around 40F.
My first reaction is that some relay is getting stuck somewhere, and powering down the rig resets it
I am an experienced electronics geek with good troubleshooting skills, and have the proper tools to do measurements, replace parts, etc., but this is my first furnace problem, and so I would appreciate any ideas anyone may have on what the solution might be. Thanks.