York Gas Furnace: TG8S

Re: Flashing Code 4

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.

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.)
  • Temperature rise at or above maximum --> Step 2. Check Air Filter
  • Temperature rise below maximum --> Suspect Bad Limit

2. Check Air Filter

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

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

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