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.
| Qty |
Possible Causes: |
|
Bad Limit(s) |
| 2 |
Bad Control Board |
| 1 |
Bad Blower Motor |
|
Missing or Loose Molex Plug |
|
Heat Relay Stuck On |
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