AHX/MX Air Handler - Flashing Code 4

York Air Handler: AHX / MX

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.

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 Closed ⇒ Re-attach molex plug and re-power air handler.
    • Limit Closed + Code 3 or 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)
  • At the motor, check the high voltage plug for 230VAC
  • At the motor, check for 24VAC between "C" and "Red" at motor.
    • 230VAC + 24VAC on Red + No Blower = Bad Motor
    • 230VAC + No 24VAC = Bad Control Board

4. Check Heaters

• Make sure there is no call for auxiliary heat.
  • Remove any low voltage wiring from the White and Brown wires.
• Check amperage of each heater
  • No Call for Heat + Amperage = Stuck Heat Relay or Bad Heater Board
    • Remove the low voltage wires from the heat relay and recheck amperage.
      • Amperage Goes Away = Bad Heater Board
      • No Low Voltage + Amperage = Stuck Heat Relay