York Gas Furnace: PHU
Re: Flashing Code 4
(Assuming original control board)
CODE 4 = High Limit or Spill Limit 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.
► The diagnostic code will go away once the limit switch closes.
► A Code 4 will rarely be the reason for a "No Heat" call (unless the Manual-Reset Spill Limit is tripped, the Primary Limit is bad, or the Blower Motor has failed).
Qty |
Possible Causes: |
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Dirty or Restrictive Air Filter |
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Clogged Evaporator Coil |
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Under-sized Ductwork |
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High Gas Pressure |
1 |
Bad Limit |
1 |
Bad Blower Motor/Capacitor |
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Blower Speed too Low |
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Blocked Vent (Spill Limit) |
CHECKOUT:
1. Check Limit Circuit
- Disconnect power to the furnace.
- Remove the 12-Pin molex plug from the control board.
- Check resistance through the limit circuit - Pin #7 (White) to Pin #1 (Red).
- Limit Circuit Closed → Step 2. Check Heating Operation
- Open Limit Circuit ⇒ Check resistance across each limit individually.
- Primary limits should reset automatically when the chamber cools.
- Cool Chamber + Open Primary Limit = Bad Limit
2. Check Heating Operation
- Start the furnace with a call for heat.
- Code 4 + Closed Limit Circuit = Bad Control Board
- Ensure that the burners light properly.
- Ensure that the blower motor start approximately 30 second after the burners ignite.
3. Check Blower Motor/Capacitor
- Disconnect power to the furnace.
- Check the fan motor capacitor.
- Capacitance less than 80% of rating = Bad Capacitor
- Spin the blower wheel to make sure it turns freely.
- Disconnect blower motor wiring from the control board.
- Check resistance across motor windings (White to Red, Blue and Black).
- Open Windings ⇒ Check the motor temperature.
- Open Windings + Hot Motor ⇒ Allow motor to cool, then recheck windings.
- Open Windings + Cool Motor = Broken Wire or Bad Blower Motor
- Resistance on All Speeds ⇒ Continue
- Re-connect the motor wiring to the control board.
- Re-apply power to the furnace.
- Energize a call for "Fan".
- Check for 120VAC between "Cir" and "Heat", and "Cir" and "Cool" on the control board.
- Power Present + No Blower Operation = Bad Blower Motor
- No Power Present = Bad Control Board
4. Check Temperature Rise
- With the furnace operating, check the temperature rise.
- Compare the actual temperature rise to the rated temperature rise on the furnace nameplate. (typically 40 - 70 deg.)
- Temperature rise below maximum ⇒ Suspect a Bad Limit
- Temperature rise at or above maximum ⇒
- Check Air Filter
- Check Evaporator Coil
- Check Duct Static Pressure
- Check Gas Pressure
- Recheck the Temperature Rise after making any changes.