Hi,

I have a friend with a GMC gas furnace that will not start. We are receiving a three blinking LED code that says pressure switch failed to close. We found this switch on the main air fan and discovered the fan will not run in auto or manual. We checked for 120 VAC output on the circuit board and found nothing on any of the speed terminals. Suspect a control relay on the circuit board.

Is there something we are missing?

Thanks in advance for any advice.

The pressure switch has a vacuum hose that goes to the draft inducer blower motor. If this little fan starts you have to check inducer, vent, chimney, vacuum hose, and vacuum ports for obstructions. If the motor doesn't start and has 120 volts it is probably bad or somethink got stuck in it.

The small ventilation fan does run and I do recall a vacuum hose that goes to a small black box, don't recall where the other end went.

I understand the sequence of events now, thermostat says I'am cold.
Small ventilation fan comes on.
Small black box with vacuum hose (pressure switch) says ventilation fan is running.
Burner lights.
Flame detected and preset temp made.
Main air duct blower should then come on.

Does the small ventilation fan have a filter that needs cleaning?
Is the small box with the vacuum hose the pressure switch?

Sounds like a cracked or blocked vacuum hose, dirty air in filter or stuck pressure switch.

Yes the small box with the vacuum hose is the pressure switch. There is no filter for the induced draft blower motor.