Goodman Gas Furnace: GMS9/GCS9

Re: Flashing Code 4

(Assuming original control board)

Code 4 = Limit Circuit Open

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).

CHECKOUT:

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.
  • No Blower Operation + Code 4 → Step 2. Check Control Board
  • Blower Operating + Airflow at Registers + Code 4 → Step 3. Check Limits
  • Blower Operating + Airflow at Registers + No Codes → Step 4. Check Temperature Rise
  • Blower Operating + No Airflow ⇒ Check for a Bad Blower Wheel

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  W  or Fan  G  to get the blower to Power On.
• Voltage Check
• Check for 120V between CIR-N and HEAT.
  • Power Present + Blower Operates → Step 3. Check Limits
  • Power Present + No Blower Operation = Bad Blower Motor or Capacitor
  • No Power ⇒ Check for power between CIR-N and COOL.
    • Still No Power = Bad Control Board

3. Check Limits

• There are (2) limits in the Limit Circuit that will generate a Code 4: (1) Auto-Reset Limit in the furnace heat exchanger and (1) Manual-Reset Limit (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.

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 40 - 70 deg.)
  • Temperature Rise At or Above Maximum → Step 5. Check Air Filter
  • Temperature Rise Below Maximum ⇒ Suspect Bad 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