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.
| Qty |
Possible Causes: |
| 2 |
Dirty/Restrictive Air Filter |
|
Clogged Evaporator Coil |
| 1 |
Clogged Recoup Coil/Blower Wheel |
| 1 |
Under-Sized/Restricted Ductwork |
| 2 |
High Gas Pressure |
|
Bad Limit |
|
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 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