York Air Handler: Flashing Code 4

York Air Handler: MP, MX, MV, AHP, AHX, AVY

Re: Fan Runs Continuous, Flashing Code 4

Code 4 = Extended Limit Trip

The control board in the air handler monitors the limit switch(es) in the aux heaters.

If the limit switch opens for a long duration (fault code 4), the control will energize the blower until power is removed from the control.

The control will continue to cycle the heat outputs on and off as the limit re-closes and opens.

The constant fan operation will signal the homeowner that a problem has occurred and a service call is required.

Two conditions that generate a Code 4:

• Limit switch is open and remains open with no call for heat.
• The limit switch opens and remains open for more than 90 seconds with a call for heat.
• Air Handler diagnostics will NOT provide any visual indication at the thermostat.

Checkout:

1. Check for Water on Control Board

• Especially in counter-flow applications, the control board is extremely susceptible to water damage from condensate overflow.
  • If there is water in the base of the air handler, or if the control board has obvious signs of water damage ⇒ suspect a Bad Control Board

2. Check Limits & Board

• Turn off power to the air handler.
  • It is best to turn off power at the breaker box, as this test may bring you into contact with the inlet power wiring.
• Disconnect the white molex plug from the air handler control board.
• On the plug (not the board) check resistance between the Red and Brown wires.
  • No Resistance Red to Brown = Open Limit
  • Resistance Red to Brown = Limit circuit closed ⇒ Re-attach molex plug and re-power air handler.
    • Still Code 4 = Bad Control Board

3. Check Blower Operation

• Activate a call for Fan by one of the following methods:
  • At the thermostat, turn the "Fan" switch to "On".
  • At the air handler control board, place a jumper wire between "R" and "G".
• X-13 Motor (AHX/MX)
  • Check high voltage plug for 230VAC
  • Check for 24VAC between "C" and "Red" at motor.
    • 230VAC + 24VAC on Red + No Blower = Bad Motor
    • 230VAC + No 24VAC = Bad Control Board
• V/S Motor (AVY/MV)
  • Check status light on CFM board.
    • Light On + No Fan = Bad Motor or Bad CFM Board
• PSC Motor (AHP/MP)
  • Check for 230VAC between White & Active Speed Wire to blower motor.
    • No Voltage = Bad Control Board
    • Voltage + No Blower Operation = Bad Motor or Capacitor