Carrier Gas Furnaces: 58MXA

Re: Flashing Code 31 / Fault Code 31

Diagnostic Light Sequence:  3-Short Flashes, 1-Long Flash  (- - - , —)

►On Carrier 90% Furnaces, the Pressure switch may be wired in series with a Low Gas Pressure Switch.

Therefore, a Fault Code 31 is caused by one of the following:

• Pressure Switch Open with Inducer Energized
• Low Gas Pressure Switch Open (if present)

CHECKOUT:

1. Check Furnace Operation

• Start the furnace with a call for heat.
  • If power is present on "W" at power-up, the control will run the blower for 90 seconds and flash a Code 12.
• Inducer motor should start and run.
  • No Inducer + Code 31 → Step 2. Check Inducer Motor
  • Inducer Runs + Code 31 → Step 3. Check Pressure Switch Circuit

2. Check Inducer Motor

• With a call for heat, check for 115VAC between the Black and White wires of the 3-pin plug for the inducer motor.
  • No Power = Bad Control Board
  • Power + No Inducer = Bad Inducer / Capacitor or Blocked Inducer Wheel ⇒ Remove inducer and check wheel.

3. Check Pressure Switch Circuit

• Check for 24VAC between "24V Com" and the Orange wire on Pin #3 of the 9-pin plug.
  • No Power = Bad Control Board
• Check for 24VAC between "24V Com" and the Yellow wire on Pin #4 of the 9-pin plug.
  • Power on Orange Pin #3 + Power on Yellow Pin #4 + Code 31 = Bad Control Board
  • Power on Orange Pin #3 + No Power on Yellow Pin #4 = Open Pressure Switch Circuit ⇒ Check the following Safeties:
    • Pressure Switch
    • Check for 24VAC between "24V Com" and both sides of the Pressure Switch.
      • Power on Both Sides = Switch Closed ⇒ Check the Low Gas Pressure Switch
      • Power on One Side but Not the Other = Open Switch ⇒ Check Pressures (below)
        • Use a pressure gauge to check the differential pressure with the inducer operating. (Trip Point = 1.80" w.c.)
          • Differential Pressure Well Above Rating + Open Switch = Bad Pressure Switch
          • Differential Pressure Near or Below Rating ⇒ Check the inducer and combustion box negative pressures individually.
            • Low Inducer Pressure = Vent/Vent Motor Problem
            • High Combustion Box Pressure = Intake/Combustion Air Problem
    • Low Gas Pressure Switch
    • Check for 24VAC between "24V Com" and both sides of the Low Gas Pressure Switch (if used).
      • Power on Both Sides = Switch OK
      • Power on One Side Only = Open Switch ⇒ Check Gas Pressure