Goodman Gas Furnace: GMS8/GMD8/GMH8

Re: Flashing Code 4

If the limit control opens, the burners will be de-energized and the air circulation and vent blower will be powered until the limit closes.

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

► The diagnostic code will go away and operation will resume once the limit switch closes.

► As long as the blower is operating correctly, a Code 4 will rarely be the reason for a "No Heat" call, since the furnace will re-fire once the limit cools and re-closes.

CHECKOUT:

1. Check Blower Operation

• Start the furnace with a call for heat.
  • The blower motor should start withing 30 seconds of the burners firing.
    • No Blower Operation + Code 4 → Step 6. Check Blower Motor/Blower Speed
    • Blower Motor Starts & Runs Correctly → Step 2. Check Temperature Rise

2. 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 3. Check Air Filter
  • Temperature Rise Below Maximum ⇒ Suspect Bad Limit

3. Check Air Filter

• Remove air filter and recheck temperature rise.
  • Temperature rise drops into normal range = Clogged or Restrictive Air Filter
  • Temperature rise still too high → Step 4. Check Gas Pressure

4. Check Gas Pressure

• Check the manifold gas pressure.
  • 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 5. Check Duct Static Pressure
• Pressure Above Maximum ⇒ Adjust Gas Pressure & Re-check Temperature Rise

5. 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).
  • Normal Static = 0.5" w.c. or Below → Step 6. Check Blower Motor/Blower Speed
  • 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

6. Check Blower Motor/Blower Speed

• Check the blower motor speed wire attached to the "HEAT" output of the control board.
  • Red = Low
  • Orange = Med Low
  • Blue = Med Hi
  • Black = Hi
• Check for 120VAC power between the HEAT terminal and NEUTRAL.
  • No Power = Bad Control Board ⇒ Try moving the HEAT wire to the EAC terminal for temporary operation.
  • Power + No Blower Operation = Bad Blower Motor/Capacitor
  • Power + Blower Operates Correctly ⇒ Continue
• If possible, increase the Heat blower speed and Re-Check Temperature Rise