PHU - Flashing Code 4

Article ID: 346
Last updated: 9 Nov, 2022

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:
Dirty or Restrictive Air Filter
Clogged Evaporator Coil
Under-sized Ductwork
High Gas Pressure
1 Bad Limit
1 Bad Blower Motor/Capacitor
Blower Speed too Low
Blocked Vent (Spill Limit)


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 RedBlue 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.
This article was:   Helpful | Not helpful
Report an issue
Article ID: 346
Last updated: 9 Nov, 2022
Revision: 10
Access: Public
Views: 78
Comments: 0