Carrier 90% Gas Furnace: 350MAV

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 failed to close or re-opened after closing.
• Low Gas Pressure Switch Open

CHECKOUT:

1. Check Furnace Operation

• Start the furnace with a call for heat.
  • It is common for a Carrier furnace to run the blower for 90 seconds and flash a Code 12 on start-up.
• Inducer motor should start and run.
  • Code 31 but inducer motor never runs = Bad Inducer / Capacitor or Bad Control Board → Step 2. Check Inducer Motor
  • Inducer Runs but flashes a Code 31 → Step 3. Check Safety 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

3. Check Safety Circuit

• Check for 24VAC between "24V Com" and the Orange wire on Pin #4 of the 9-pin plug.
  • No Power = Bad Control Board
• Check for 24VAC between "24V Com" and the Yellow wire on Pin #2 of the 9-pin plug.
  • Power on Pin #4 + Power on Pin #2 + Code 31 = Bad Control Board
  • Power on Pin #4 + No Power on Pin #2 = Open Circuit ⇒ Check 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 Only = 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 near or below the trip point ⇒ Check the inducer and combustion box negative pressures individually.
        • ► L​​​​​ook into the pressure ports to make sure they are not clogged.
          • Low Inducer Pressure = Vent or Vent Motor Problem
          • High Combustion Box Pressure = Intake 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 but Not the Other = Open Switch ⇒ Check Gas Pressure