TUH/TDH - Flashing Code 3

Trane Gas Furnace: TUH1/TDH1

Re: Flashing Code 3

Code 3 = Pressure switch fault

This furnace uses a dual-hose (differential pressure) pressure switch.  You need to monitor both hoses at the same time to see actual working pressure.

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 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.

CHECKOUT:

1. Check Control Board

• Start the furnace with a call for heat.
• Ensure that power is present on "W" by checking for 24VAC between "W" and "C".
  • No Power = No Call for Heat
  • Power = The Inducer motor should start within 15 seconds.
    • Inducer Motor Starts → Step 2. Check Inducer
    • Inducer Motor Does Not Start + Code 3 ⇒ Check for 115V power to inducer.
      • Power + No Inducer → Step 2. Check Inducer
      • No Power = Check for a Stuck Pressure Switch

2. Check Inducer

• Observe and listen to the inducer motor.
  • Inducer motor runs normally → Step 3. Check Pressure Switches
  • Inducer motor hums but does not start = Bad Inducer Motor/Capacitor or Item in Inducer ⇒ Remove inducer & check
  • Sound of Water "Slurshing" = Water in Inducer ⇒ Remove Inducer, pour out water, re-install inducer → Step 4.  Check Condensate Drainage
• Listen for sounds of water in the vent piping (near the furnace and at the vent termination).
  • If water is gurgling in the vent, look for sags or low spots in the vent piping that will hold water.
    • The vent piping must have a constant slope back toward the furnace.

3. Check Pressure Switches

• Make sure there are no sags in the pressure switch hoses that could hold water.
• Start the furnace with a call for heat.
• Switch Contacts
  • With the wires attached to the switch, check for 24VAC between each terminal of the switch and Common.
    • 24VAC On Both Sides = Switch Closed 
    • 24VAC On One Side Only = Open Switch ⇒ Use a pressure meter to check the Switch Pressure (below).
• Switch Pressure
  • Remove the hoses from the pressure.
  • Inspect the pressure switch hose for a:
    • Loose hose
    • Dry-rotted, cracked hose
    • Clogged hose
  • Connect both hoses to a pressure meter.
  • Compare actual pressure to the pressure rating on the switch.
    • Pressure Near or Below Rating =
      • Bird or Water in Inducer
      • Clogged or Restricted Vent
      • Clogged or Restricted Intake
    • Pressure Well Above the Rating + Open Switch = Bad Switch or Water in Switch
      • ​​​​Water in Switch is usually caused by a mis-run pressure switch hose - should loop up so water drains back to the inducer (not the pressure switch).

4. Check Condensate Drainage

• Check the drain hose from the collector box.
  • Make sure all drain hoses are clear and have a downward slope.
• In extreme cold conditions be cautious of Frozen Drains.