York Gas Furnace: P3DH

Re: Flashing Code 4

CODE 4 = High Limit or Blower Limit Open

There are three limits on this furnace:

• Primary Limit
• Auxiliary Limit
• Blower Limit

If any of the three limit controls open, 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.

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.
    • Blower limit must be manually reset.
    • Primary limits should reset automatically when the chamber cools.
      • Cool Chamber + Open Primary Limit = Bad Limit
      • Tripped Blower Limit ⇒ Suspect Blower Motor Problem or Loss of Power During a Heat Cycle

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.
  • No Blower Operation → Step 3. Check Blower Motor/Capacitor
  • Blower Operates → Step 4. Check Temperature Rise

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 (A clogged evaporator coil typically = a clogged re-coup coil.)
    • Check Duct Static Pressure
    • Check Gas Pressure
      • Recheck the Temperature Rise after making any changes.

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
    • Low Return Static + Low Supply Static = Low Blower Speed, Clogged Blower Wheel or Clogged Re-Coup Coil