AHX/MX Air Handler - Fan Runs Continuous - No Faults

York Air Handler: AHX/MX

Re: Fan Running Continuous - No Fault Codes

The customer reports that the indoor blower runs continuous.

► This is normally associated with a Code 3 on the air handler board, but in this case there is no fault indication on the board.

Checkout:

1. Check the Thermostat

• At the thermostat, make sure the System is turned "Off" and the Fan is set to "Auto".
• Check to see if the indoor blower shuts off within 60 seconds.
  • Fan Continues to Run → Step 2. Check Low Voltage

2. Check Low Voltage

• At the air handler, remove the blower cabinet door.
  • ► If the blower is running, be careful with the blower door insulation (it will try to pull off).
• Check for 24VAC between "C" and "G", "Y", & "W".
  • No Power → Step 3. Check Control Board
  • Power Present ⇒ Continue
    • Remove the powered low voltage wire from the air handler board.
      • Blower Continues to Run → Step 3. Check Control Board
      • Blower Shuts Off ⇒ Continue
        • Check for 24VAC between "C" and the removed wire.
          • Power Present ⇒ Pull the thermostat off of the subbase and check (at the subbase) for power between "C" and the powered wire.
            • No Power = Bad Thermostat
            • Power Present ⇒ Remove the powered wire from the subbase and check for power between "C" and the wire.
              • Power Present = Shorted Low Voltage Wire
              • No Power ⇒ Check for 24VAC between "C" and the now empty subbase terminal.
                • Power Present = Bad Subbase

3. Check Control Board

• Confirm that there is no fault code on the control board.
  • ​​​​​​​Cycle high voltage power at the air handler just to make sure there was no lockout.
• Remove the two low voltage wires at the motor plug (should be Red and White).
  • Blower Continues to Run = Bad X-13 Motor
  • Blower Stops = Bad Control Board
    • ​​​​​​​​​​​​​​Especially in counter-flow applications, the control board is extremely susceptible to water damage from condensate overflow.
      • ► Look for water in the base of the air handler, or obvious signs of water damage.