Goodman Gas Furnace: GMP

Re: Flashing Code 4

(Assuming original control board)

If the limit control opens, the burners will be de-energized and the air circulation and vent blower will be turned on until the limit closes. The diagnostic light code for this is four short flashes followed by a pause.

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

► A Code 4 usually isn't a reason for a "No Heat" call. The furnace will re-fire once the limit cools and re-closes.

CHECKOUT:

1. Check Operation

• Set the thermostat to call for heat.
  • Furnace Starts Normally (No Diagnostic) --> Step 2. Check Blower Operation
• Check for 24VAC between the Red wire on Pin #3 and the Blue wire on Pin #6 on the control board.
  • No Power = No High Voltage Power or Bad Transformer
  • Power Present --> Continue
• Check for 24VAC between "R" and Gnd on the low voltage wiring.
  • No Power on "R" = Blown Low Voltage Fuse or Open Limit
    • Check for 24VAC power from both side of the Low Voltage Fuse to Gnd.
      • Power on one side but not the other = Blown Low Voltage Fuse
      • Power on Both Sides + No Power on "R" = Bad Control Board
  • Power on "R" --> Continue
• Check for 24VAC between "W" and Gnd.
  • No Power = No Call for Heat from the Thermostat
  • Power on "W" + Code 4 = Open Limit or Bad Control Board
    • Check for 24VAC from Pin #5 and Pin# 9 on the control board to Gnd.
      • Power on both + Code 4 = Bad Control Board
      • Power on one but not the other = Open Limit

2. Check Blower Operation

• After the burners fire, ensure that the blower motor starts (up to 30 second delay).
  • Blower Starts and Runs Normally --> Step 3. Check Temperature Rise
  • Blower Motor Does No Start --> Continue
• Check Capacitor
  • Turn off all power to the furnace unit.
    • Use a multi-meter to confirm that voltage is not present.
  • Remove capacitor wires (short before touching)
  • Move Multi-Meter leads to COM and MFD.
  • Set Meter to 200 MFD.
  • Place one lead on each side of the capacitor. 
  • Confirm MFD reading with capacitor rating.
    • Less than 80% of rating = Bad Capacitor
  • SWITCH METER LEADS BACK TO COM AND V BEFORE PROCEEDING!
• Check Blower Motor
  • Re-apply power to the furnace.
  • Set the thermostat to call for heat.
  • After the burners ignite, wait 30 seconds then check power to the blower motor.
    • Check for 120VAC from the control board Neutral block to the control board "Heat" fan output.
      • Power but No Motor Operation = Bad Blower Motor
      • No Power = Bad Control Board

3. Check Temperature Rise

• Start the furnace with a call for heat.
• Allow the furnace to operate for at least 5 minutes.
• 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.
  • Temperature rise at or above maximum + Limit Trips --> Step 2. Check Air Filter
  • Temperature rise below maximum + Limit Trips = Bad Limit

4. Check Air Filter

• Remove air filter and recheck  the temperature rise.
  • Temperature rise drops into normal range = Clogged Air Filter
  • Temperature rise still too high --> Step 3. Check Gas Pressure

5. 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 4. Check Blower Speed
    • Pressure above maximum --> Adjust Gas Pressure & Re-check Temperature Rise

6. 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

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