My furnace recently starts acting weird. After running up to satisfied temperature, the thermostat stops normally, then the blower stops 30 seconds later as usual, but the draft motor sometimes keep on running forever. Turn off then on the power to furnace obviously reset it to normal, removing the thermostat from the wall won't stop it, but switch the thermostat to "FAN ON" then switch to "FAN AUTO" will stop it.

It's hard to troubleshoot since the problem is not consistent, it happens every 2-4 days, sometimes after a long run, sometimes after a shot run.

I check the pressure switch, got 24V when the furnace is running, but got nothing when only the draft motor runs. The drain is not blocked. I just don't know what control the draft motor. I also cleaned the flame sensor, although it doesn't seem to be related.

I can attach some photos of the furnace and the wiring diagram if needed.

Any help is appreciated, thanks.