GMH95 / GCH95 - Flashing Code 4

Goodman Gas Furnace: GMH95/ GCH95

Re: Flashing Code 4

Code 4 = Main limit switch (auto-reset) has opened its normally closed contacts, or the Blower Auxiliary Limit (manual-reset) has tripped.

• The control will operate the supply air blower and inducer while the open limit condition exists.

CHECKOUT:

1. Check Limit Circuit

• While the furnace is Flashing Code 4, check for 24VAC across the limit circuit.
  • Check for 24VAC between "C" Common and Terminal #1 of the 12-Pin Plug.
  • Check for 24VAC between "C" Common and Terminal #7 of the 12-Pin Plug.
    • Power on Both Terminals + Code 4 = Bad control Board
    • Power on One Terminal Only = Open Limit Circuit ⇒ Check Primary Limit (below)
      • Primary Limit
      • ​​​​​​​Check for 24VAC between "C" Common and both sides of the Primary Limit.
        • Power on Both Sides = Limit OK ⇒ Check Aux Limit (Below)
        • Power on One Side Only = Open Limit
          • ​​​​​​​Open Limit + Cool Chamber = Bad Limit
          • Open Limit + Hot Chamber → Step 2. Check Blower Operation
      • Aux Limit
      • Check for 24VAC between "C" Common and both sides of the Auxiliary Limit (blower compartment).
        • Power on One Side Only = Open Limit ⇒ Reset Limit and → Step 2. Check Blower Operation

2. Check Blower Operation

• While the furnace is operating or Flashing Code 4, check to see if the blower motor is operating.
  • Blower Motor Running Normally → Step 3. Check Temperature Rise
  • Blower Motor Not Running ⇒ At the furnace control board, check for 120VAC between "CIR-N" and "LO HEAT-H", then between "CIR-N" and "HI HEAT-H".
    • 120VAC Present + No Blower = Bad Blower Motor or Capacitor
    • No 120V Present = Bad Control Board

3. Check Temperature Rise

• Start the furnace with a call for heat.
• Check the temperature rise across the furnace.
• 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 4. Check Air Filter
  • Temperature Rise Below Maximum but Limit Trips ⇒ Suspect Bad Limit

4. 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 5. Check Gas Pressure

5. Check Gas Pressure

• De-energize the call for heat and allow the furnace to cool.
• Energize a call for heat.
• Check the manifold gas pressure.
• 1st Stage Heat
  • Natural Gas = 1.7 to 2.3" w.c.
  • LP Gas = 6" to 8" w.c. 
• 2nd Stage Heat
  • Natural Gas = 3.2 to 3.8 " w.c.
  • LP Gas = 9" to 11" w.c. 
    • Pressure Above Maximum ⇒ Adjust Gas Pressure & Re-check Temperature Rise
    • Pressure Below Maximum + Still Over-Heating → Step 6. Check Blower Speed

6. Check Blower Speed

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

7. 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