York 90% Gas Furnace: P*MPD

Re: Flashing Code 3

Code 3 = Furnace Pressure Switch failed to close on a call for heat.

This gas furnace has a single pressure switch which monitors for proper venting.

CHECKOUT:

1. Check Control Board

• Start the furnace with a call for heat.
  • The Inducer motor should start within 15 seconds.
    • Inducer motor does not start → Step 2. Check Inducer Motor
    • Inducer motor starts and runs normally → Step 3. Check Pressure Switch

2. Check Inducer Motor

• Start the furnace with a call for heat.
• Observe and listen to the inducer motor.
  • Inducer motor hums but does not start:
    • Bad Inducer Motor or Capacitor
    • Item in Inducer ⇒ Remove inducer & check
  • Inducer motor does not attempt to start & shows Code 3 ⇒ Check for 115V power to inducer.
    • Power + No Inducer = Bad inducer Motor
    • No Power = Bad Control Board

3. Check Pressure Switch

• Ensure the pressure switch hose is properly attached at both ends.
• Switch Contacts
• With the wires attached to the switch, check for 24VAC between each terminal of the switch and Common.
  • 24VAC On One Side Only = Open Switch ⇒ Use a pressure meter to check the Switch Pressure (below).
  • 24VAC On Both Sides = Switch Closed 
    • Switch Closed + Code 3 ⇒ Check Power at the control board plug
• Switch Pressure
• Remove the pressure hose from the Inducer Motor.
• Connect a pressure meter to the Inducer Motor pressure port.
• Compare actual pressure to the pressure rating on the switch.
  • Pressure Near or Below Rating = Clogged or Restricted Vent or Bad Vent Blower Wheel ⇒ Remove and Check Blower
  • Pressure Well Above the Rating + Open Switch = Bad Switch