TUX1/TDX1 - Flashing Code 3

Article ID: 387
Last updated: 13 Mar, 2023

Gas Furnace: TUX1/TDX1

Re: Flashing Code 3

Code 3 = Pressure switch fault

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

  • Pressure switch contacts "Closed" when the inducer motor is commanded OFF.
  • Pressure switch contacts "Open" when the inducer motor is commanded ON.

► If the 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.

Qty Possible Causes:
2 Bad Inducer Motor / Capacitor
2 Clogged Drain
1 Bad Pressure Switch
1 Bad Control Board
Vent Pipe Problem
Intake Pipe Problem
ADPS Switch Set Too Low


1. Check Sequence

  • Activate a call for heat from the thermostat (confirm 24VAC between "W" and "C")

2. Check Inducer Motor

  • With "W' energized, check for 120V power at the inducer motor wiring plug:
    • Pressure Switch Open + No Power = Bad Control Board
    • Power + No Fan = Bad Inducer Motor or Capacitor

3. Check Pressure Switch

  • Check Switch Contacts
  • With "W" energized and the Inducer Running:
  • Check for 24VAC from both sides of the pressure switch to "C".
    • Power on Both Sides = Switch Closed
    • Power on One Side Only = Switch Open
      • Inducer Running + Switch Closed + Code 3 = Bad Control Board
  • Check Pressure
  • Hook-up a pressure gauge to the pressure switch hoses.
    • On a 2-hose switch, connect the (-) side to the inducer and the (+) to the burner box to read "differential" pressure.
  • Monitor the system pressure as the furnace operates.
    • Pressure Well Above Rating + Open Switch = Bad Pressure Switch
    • Pressure Near or Below the Switch Rating = Vent/Intake Issue ⇒ Check the negative pressure at the inducer and burner box individually.
      • Inducer Negative Pressure Low = Vent or Drain Problem
      • Burner Box Negative Pressure High = Intake Problem

4. Check Condensate Drain

  • Listen for the sound of gurgling water in the inducer.
  • Disconnect and check the drains from the inducer and/or collector box.

5. ADPS Switch Setting

  • The ADPS switch will sometimes fail to re-open when set for the lowest setting.
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Article ID: 387
Last updated: 13 Mar, 2023
Revision: 20
Views: 49
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