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350MAV - Flashing Code 31/Fault Code 31

Article ID: 388
Last updated: 04 Mar, 2021

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

Qty Possible Causes:
Blocked Vent
1 Bad Pressure Switch
1 Bad Inducer Motor
1 Clogged Pressure Port
Clogged / Frozen Drain
Item Stuck in Inducer Motor
Bad Ignition Control
Low Gas Pressure

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.

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
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Article ID: 388
Last updated: 04 Mar, 2021
Revision: 19
Views: 7814
Comments: 0