Trane Gas Furnace: TUD/TDD

Re: Flashing Code 4

Code 4 = Limit switch open

There are 4 limits wired in series: a Reversed Flow Limit in the blower compartment (auto-reset), two Roll-Out Limits (manual-reset), and a Primary Limit (auto-reset).

CHECKOUT:

1. Check the Limit Circuit

• At the control board 12-pin plug, check for 24VAC between HLO Pin #1 (Yel) to "C".
• At the control board 12-pin plug, check for 24VAC between HLI Pin #7 (Yel) to "C".
  • No Power on Either Pin = No Control Power or Bad Control Board
  • Power on HLO Pin #1 only = Limit Circuit Open → Step 2. Check Limits
  • Power at both Pins #1 & #7 = Limit Circuit Closed
    • Limit Circuit Closed + Code 4 = Bad Control Board

2. Check Limits

• Reversed Flow Limit
• Check for 24VAC at both sides of the limit (located on the blower housing).
  • Power on One Side Only = Limit Open → Step 3. Check Blower Motor
  • Power on Both Sides ⇒ Continue
• Roll-Out Limits
• Check for 24VAC at both sides of each roll-out limit (located above the burners).
  • Power on One Side Only = Limit Open → Step 4. Check Burners/Heat Exchanger
  • Power on Both Sides ⇒ Continue
• Primary Limit
• Check for 24VAC at both sides of the primary limit (located on the heat exchanger plate).
  • Power on One Side Only = Limit Open → Step 5. Check Temperature Rise

3. Check Blower Motor

• If the limit circuit is open and there is a call for heat (24VAC on "W") the inducer motor and the blower motor should be running.
  • Blower Operating + Open Reversed Flow Limit ⇒ Suspect a Bad Reversed Flow Limit
  • Blower Motor Not Operating ⇒ ​​​​​​​Check for 120VAC going to the blower motor between control board "CIR-N" & "HEAT".
    • ​​​​​​​Code 4 + No Power = Bad Control Board
    • Power Present + No Blower Operation ⇒ 
      • Turn off power to the furnace
      • Check to​​​​​​​ see if the motor spins freely
      • Check motor temperature (a hot motor may be off on internal overload)
      • Check the blower motor capacitor

4. Check Burners/Heat Exchanger

• Reset the tripped roll-out limit by pressing the button in the center of the limit switch.
• Activate a call for heat (if not already calling).
• Code 4 should go away and an ignition sequence should start.
• Watch burners for proper ignition.
• Continue to watch burners when the blower motor starts.
  • ​​​​​​​Roll-out or significant change in burners = Bad Heat Exchanger​​​​​​​

5. Check Temperature Rise

• Start the furnace with a call for heat.
• Check and record the return air temperature near the furnace (at the filter if possible) with the blower on.
• Check the supply air temperature (in a straight duct run near the furnace) as the furnace operates.
• Compare the actual temperature rise to the rated temperature rise on the furnace nameplate. (typically 40 -70 deg.)
  • Temperature Rise Below Maximum + Code 4 ⇒ Suspect Bad Primary Limit
  • Temperature Rise Near or Above Maximum = Furnace Over-Heating ⇒
    • Check Air Filter
    • Check Gas Pressure
    • Check Blower Speeds
    • Check Recoup Coil
    • Check Duct Static Pressure