Goodman Gas Furnace: GMNT
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 will rarely be the reason for a "No Heat" call. The furnace will re-fire once the limit cools and re-closes.
Qty |
Possible Causes: |
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Dirty Air Filter |
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Clogged Evaporator Coil |
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Under-sized Ductwork |
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High Gas Pressure |
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Bad Limit |
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Blower Speed too Low |
CHECKOUT:
1. 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.)
2. Check Air Filter
- Remove air filter and recheck temperature rise.
3. 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
4. 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
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).
- 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