GCSS92 - Flashing Code 4

Article ID: 726
Last updated: 14 Sep, 2023

Goodman Gas Furnace: GCSS92

Re: Flashing Code 4

Code 4 = Limit Circuit Open

The Limit Circuit consists of two Manual-Reset* auxiliary limits (on each side of the blower housing) and one Auto-Reset primary limit (above the burners).

    * The diagram and installation manual show Manual-Reset limits, but we have seen Auto-Reset limits in field service calls.

If the Limit Circuit opens, the burners will be de-energized and the air circulation and vent blower will be turned on until the Limit Circuit closes.

The diagnostic light code for this is four short flashes followed by a pause.

► The diagnostic code will go away once the Limit Circuit closes (not a Hard Lockout).

Qty Possible Causes:
Blower Failure
Dirty Air Filter
Bad Control Board
Clogged Evaporator Coil
Under-sized Ductwork
High Gas Pressure
Bad Primary Limit
1 Bad Auxiliary Limit
Blower Speed too Low


1. Check Blower Operation

  • When the furnace is Flashing Code 4, the indoor blower and inducer motor should be running.
  • Ensure that the blower motor is operating and airflow is coming from the registers.

2. Check Control Board

  • If the furnace is still in a Code 4 lockout, the blower should be operating, so move on to the Voltage Check.
  • If the Code 4 goes away, activate either a call for Heat  or Fan  G  to get the blower to Power On.
  • Voltage Check
  • Check for 120V between NEUTRAL and HEAT-H.
    • Power Present + Blower Operates → Step 3. Check Limits
    • Power Present + No Blower Operation = Bad Blower Motor or Capacitor
    • No Power ⇒ Check for power between NEUTRAL and COOL.
      • Still No Power = Bad Control Board

3. Check Limits

  • There are (3) limits in the Limit Circuit that will generate a Code 4: (1) Auto-Reset Limit in the furnace heat exchanger and (2) Manual-Reset Limits (In the Blower Deck on Upflow, or On the Blower Housing on Counterflow).
  • Check the Limit Circuit
  • Check for 24V between "C" and Pin #1  Pink  of the 12-pin plug on the circuit board.
  • Check for 24V between "C" and Pin #7  Blue  of the 12-pin plug on the circuit board.
    • Power on Both Pins = Limit Circuit Closed 
      • Limit Circuit Closed + Code 4 = Bad Control Board
    • Power on One Pin Only = Limit Circuit Open
      • Check for 24V between "C" and both sides of each limit to determine which is Open.
        • An open Auxiliary Limit is intended to protect against a Blower Failure

4. 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 35 - 65 deg.)
    • Temperature Rise At or Above Maximum → Step 5. Check Air Filter
    • Temperature Rise Below Maximum ⇒ Suspect a Bad Primary Limit

5. Check Air Filter

  • Remove air filter and recheck temperature rise.
    • Temperature Rise Becomes Normal = Clogged or Restrictive Air Filter
    • Temperature Rise Still Too High → Step 6. Check Gas Pressure

6. Check Gas Pressure

  • De-energize the call for heat and allow the furnace to cool.
  • Turn off power to the furnace and close the gas safety shut-off.
  • Hook-up a gas pressure gauge to the manifold (leaving) test port on the gas valve.
  • Open the gas safety shut-off valve and re-apply furnace power.
  • Activate a call for heat and monitor the manifold gas pressure when the gas valve opens.
    • Natural Gas = 2.5" w.c. minimum to 3.5" w.c. maximum
    • LP Gas = 9" w.c. minimum to 11" w.c. maximum
      • Pressure Below Maximum → Step 7. Check Blower Speed
      • Pressure Above Maximum ⇒ Adjust Gas Pressure & Re-check Temperature Rise

7. Check Blower Speed

  • Check the blower motor speed wire attached to the "Heat" output of the control board.
    • Red = Low
    • Yellow = Med Low
    • Blue = Med Hi
    • Black = Hi
  • If possible, increase the Heat blower speed and Re-Check Temperature Rise

8. Check Duct Static Pressure

  • Check the return static pressure (in the blower door if possible).
  • Check the supply static pressure (between the furnace and evaporator coil if possible).
    • High Duct Static = Above 0.5" w.c.
      • High Return Static + Low Supply Static = Restricted Return Ductwork
      • Low Return Static + High Supply Static = Clogged Coil or Restricted Supply Ductwork
        • Compare static on both sides of evaporator coil.
          • High Static Entering Coil + Low Static Leaving Coil = Clogged Coil
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Article ID: 726
Last updated: 14 Sep, 2023
Revision: 8
Access: Public
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