Gas Furnace: TUX

Re: Flashing Code 4

Code 4 = Open Limit Circuit

There are three limits in the limit circuit:

• Main Limit (TCO-A) - Automatic-reset limit for the heat exchanger.
• Roll-Out Limit - Manual-reset limit mounted above burners.
• Auxiliary Limit (TCO-B) - Automatic-reset limit, Usually mounted on the blower compartment.

CHECKOUT:

1. Check Limit Circuit 

• Turn off power to the furnace.
• Remove the 9-pin plug from the furnace control board.
• At the plug (not the board), check resistance between pin 3 (YL/1) and pin 4 (YL/5).
  • No Resistance = Limit Circuit Open → Step 2. Check Primary Limit
  • Resistance = Limit Circuit Closed ⇒ Re-attach 9-pin plug, re-apply power to the furnace, and cycle a call for heat.
    • Normal Heat Cycle → Step 5. Check Blower Operation
    • Still Code 4 = Bad Control Board

2. Check Primary Limit

• Attempt to perform this test without removing wires from the limit (moving wires may cause the limit to reset).
• Check resistance between the two terminals (YL/1) & (YL/2) of the primary limit.
  • Resistance = Limit Closed → Step 3. Check Roll-Out Limit
  • No Resistance = Limit Open
    • Limit Open + Cold Chamber = Bad Primary Limit
    • Limit Open + Hot Chamber → Step 5. Check Blower Operation

3. Check Roll-Out Limit

• Check resistance between the two terminals (YL/2) & (YL/3) of the roll-out limit.
  • Resistance = Limit Closed → Step 4. Check Auxiliary Limit
  • No Resistance = Limit Open
    • ​​​​​​​Press reset button on the top of the limit & recheck resistance.
      • ​​​​​​​Resistance ⇒ Check for heat exchanger issues
      • No Resistance = Bad Roll-Out Limit

​​​​​​​4. Check Auxiliary Limit

• Access the blower compartment.
• Attempt to perform this test without removing wires from the limit (moving wires may cause the limit to reset).
• Check resistance across the the terminals (YL/3) & (YL/4) of the auxiliary limit.
  • Resistance = Limit Closed ⇒ Recheck limit circuit at 9-pin plug (Step 1).
    • ​​​​​​​Limit Circuit Closed → Step 5. Check Blower Operation
    • Limit Circuit Open ⇒Check for bad wiring between 9-pin plug and limits.
  • No Resistance = Limit Open
    • Limit Open + Cold Chamber = Bad Auxiliary Limit
    • Limit Open + Hot Chamber → Step 5. Check Blower Operation

​​​​​​​5. Check Blower Operation

• Remove power from the furnace.
• Access the blower compartment.
• Remove the wires from the blower motor capacitor.
• Check capacitor.
  • Bad ⇒ Replace & Continue
  • Good ⇒Continue
• ​​​​​​​Close blower access panel.
• Cycle the furnace through a call for heat & monitor blower motor operation in 1st & 2nd Stage Heat
  • ​​​​​​​Blower Starts & Runs → Step. 6. Check Temperature Rise
  • No Blower Operation ⇒ At the control board, check for 120VAC power between [LO HEAT] & [CIR-N], then between [HI HEAT] & [CIR-N].
    • ​​​​​​​Power + No Blower Operation = Bad Blower Motor
    • No Power = Bad Control Board

​​​​​​​6. 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 (on both 1st & 2nd Stage Heat).
• Compare the actual temperature rise to the rated temperature rise on the furnace nameplate. (typically 35 - 65 deg.)
  • Temperature Rise Near or Above Maximum ⇒ Check for low airflow (Dirty Filter, Dirty Coil, Low Blower Speed, etc.)
  • Temperature Rise Below Maximum + Code 4 ⇒ Suspect Bad Primary Limit