TG9S/GY9S - Flashing Code 4

York Gas Furnace: TG9S/GY9S

Re: Flashing Code 4

Code 4 = Limit switch open

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.

► 4 Limit Trips during a single heat call will generate a 1-Hour Soft Lockout.

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 35 - 65 deg.)
  • Temperature Rise Near or Above Maximum → Step 2. Check Air Filter
  • Temperature Rise Below Maximum + Code 4 ⇒ Suspect Bad Limit

2. Check Air Filter

• Remove the air filter and recheck the temperature rise.
  • Temperature Rise Drops into Normal Range = Clogged, Restrictive or Under-Sized Filter
  • Temperature Rise Still too High → Step 3. Check Gas Pressure

3. 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 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 (between the filter & the blower).
• Check the supply static pressure.
  • High Duct Static = Above 0.5" w.c.
    • High Return Static + Low Supply Static = Restricted Return Ductwork or Filter ⇒ Check the static pressure on both sides of the air filter.
      • Drastic difference = Clogged, Restrictive or Under-Sized Filter
    • Low Return Static + High Supply Static = Restricted Supply Ductwork
    • Low Supply Static + Low Return Static = Clogged Evaporator Coil, Blower Wheel, Recoup Coil or All Three