Gas Furnace: TUX/TDX 2-Stage

Re: Flashing Code 3 / Inducer Fan Not Running

Code 3 = Pressure switch fault

There are two pressure switches on this furnace, one for low speed fan and one for high speed.

A Code 3 will result from either of the following conditions:

• Pressure switch contacts "closed" when they should be open.
• Pressure switch contacts "open" when they should be closed.

► If either pressure switch is in the "closed" position when a call for heat is initiated, the control board will flash a Code 3 and NOT power the inducer motor.

CHECKOUT:

1. Check Controller

• Activate a call for heat from the thermostat (confirm 24VAC between "W" and "C")
  • No Inducer Fan and Code 3 = Switch Stuck Closed, Bad Inducer Motor/Capacitor or Bad Control Board--> Step 2. Check Inducer Motor
  • Inducer Fan Starts and Control Board Flashes Code 3 after several seconds = Switch Stuck Open --> Step 3. Check Pressure Switch Contacts

2. Check Inducer Motor

• With "W' energized, check for 120V power at the inducer motor wiring plug:
  • Between "White" and "Red" (Low Speed Fan)
  • Between "White" and "Black" (High Speed Fan)
    • No Power = Pressure Switch Stuck Closed or Bad Control Board --> Step 3. Check Pressure Switch Contacts
    • Power but No Fan = Bad Inducer Motor or Capacitor

3. Check Pressure Switch Contacts

• With "W" energized:
• Check for 24VAC from both sides of each pressure switch to "C".
  • Power on both sides = Switch Closed --> Remove one wire from the switch and re-check power to inducer.
  • Power on only one side = Switch Open
• Both Switches Open and No Power to Inducer = Bad Control Board

4. ADPS Switch Setting

• The ADPS switch will sometimes fail to re-open when set for the lowest setting.