I have a HEIL gas furnace model #NCC5075BHB2. The combustion fan (exhaust blower) cycles on and off for about 20 times after a heating cycle. I have checked, roll out switches, and limit switches; all are closed (have continuity), the pressure switch is open and closes with vacuum. If the pressure switch is jumped or a continuity tester is on it everything works correct. If any sensors are disconnected fan runs constantly. The combustion area is not hot. The only thing I have not checked is the main control board. I would appreciate any ideas on possible fixes. Thanks

Sounds like the pressure switch is still making contact all the time or it could be getting a back feed. Check the ground wires from the main panel to the switch for the furnace then to the furnace itself. I am not talking about the white neutral wire but the bare copper or green wire.

Circuit boards are very sensitive to a bad grounding condition. I have seen a bad ground cause strange things in a furnace and other items.

Thanks for the quick response. I checked the ground wires all the way to the main electrical panel and also cleaned every other connection I could find in the furnace. I still have the problem cycling on 5 sec and off 3sec. Something is starting the blower and then as soon as the pressure switch senses the change it shuts down which causes the switch to open and the cycle goes on and on. One thing that has me baffled is I have been shutting off the heater with the thermostat switch to keep it from cycling, and it finishes its 5 sec cycle and doesn't come on again. The thermostat should be nothing but an on/off switch based on temperature so if it isn't calling for heat it should be the same as being in the off position. Is it possible the thermostat is somehow causing feed back to the exhaust blower circuit? Thanks for your help; if you have any other ideas please let me know.

The way to check this is to eliminate the thermostat. Remove the heat wires and all should stop if it is a thermostat problem.

I replaced the thermostat today and everything seems to be working properly. Thanks again for the help.

Glad it worked out for you.