Carrier Furnace Code 31 [Solved]

Are you dealing with a carrier furnace code 31? If you’re then this guide is for you. I will be taking you through all possible causes for status code 31 and what you can do to troubleshoot and fix this problem.

Status code 31 on a carrier furnace indicates that the pressure switch failed to close or reopened after closing. It can also mean that the low gas pressure switch is open.

PS: Check link to the troubleshooting manual at the bottom of this guide ( from page 19)

Possible Causes For The Carrier Furnace Code 31

Possible Causes	Fix
Malfunctioning pressure switch	Replace pressure switch
Bad inducer Motor	Replace inducer motor/ assembly
Inducer wheel not properly mounted shaft	Mount it properly
• Restriction in vent pipe.• Proper vent sizing for installation.• Leakage in collector box.• Leakage in heat exchanger	Inspect each component
Malfunctioning control board	Replace control board

Troubleshooting instructions for Carrier code 31

Check inducer motor

Start by turning off power to the furnace and then open the furnace door. Depress the door switch with a tape and disconnected the R lead from terminal block

Now turn on the power and jumper the R lead to the W/W1, W2 and see if the inducer motor runs.

If the Inducer motor is not running with this test your furnaces flashes the 31 code, you may have a bad inducer motor capacitor. Or circuit control board.

To check if your inducer motor jumper the R leads to the W1 or W 2 and , check for 115VAC between the Black and White wires of the 3-pin plug for the inducer motor.

If there is no continuity, you likely have a bad furnace control board.

If you have continuity between the black and white wires but the inducer wire is not not running then you may have either a bad inducer motor/ capacitor or a blocked inducer wheel.

Check the safety circuit

Checking the safety circuit is necessary if the inducer motor is running when there is a call for heat and the furnace is flashing the 31 code error. Follow these steps:

Step 1. Check for 24VAC between “24V Com” and the Orange wire on Pin #4 of the 9-pin plugs.

If there is no continuity then, the problem is probably a bad Control Board.

Step 2. Check also for 24VAC between “24V Com” and the Yellow wire on Pin #2 of the 9-pin plug.

If you have continuity on Pin #4 and on Pin #2 and a flashing Code 31, you likely have a bad control board

If you have power on Pin #4 and No Power on Pin #2 then you have an open Circuit ⇒ Check Safeties

Check pressure switch

A defective pressure switch could also trigger the flashing code 31 on your carrier furnace. You’ll need to test to verify this. Follow these steps below

Step 1. Check for 24VAC between “24V Com” and both sides of the Pressure Switch.

If you have Power on Both Sides then the Switch is probably closed. So you’ll Check the low gas pressure switch

And if you have power on one side only then you have an open switch So you’ll need to check pressures as below.

Step 2. 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.

Look into the pressure ports to make sure they are not clogged.
If you have a Low Inducer Pressure then you likely have a vent or Vent Motor Problem
Otherwise if you have a High Combustion Box Pressure, it’s likely that this is an Intake Problem.

Check Low Gas Pressure Switch

Step 1. Check for 24VAC between “24V Com” and both sides of the Low Gas Pressure Switch (if used).

If there is continuity on Both Sides then the Switch is OK
But if there is Power on only One Side but Not the Other, you either have an Open Switch or check the gas pressure

More causes for the status code 31

Scenarios under which code 31 might occur

Preheat Phase

When you initiate a high-heat call, if the high-heat pressure switch (HPS) doesn’t close within 30 seconds, the 2-stage furnace control starts flashing Status Code 31.

It also adjusts the inducer motor speed to a slower rate for 15 seconds. After this 15-second wait, if the request for high heat is still active, the inducer speed returns to normal high-heat speed.

The flashing error code stops if the high-heat pressure switch closes.

If the switch remains open, the flashing error code persists, and the furnace continues with the high-heat cycle.

During this, the gas valve operates at a lower heat level.

Transition from Low Heat to High Heat:

If you’ve requested high heat and the high-heat pressure switch doesn’t close within 30 seconds, the 2-stage furnace control displays Status Code 31.

If the switch doesn’t close within 2 minutes after requesting high heat, the furnace control system keeps flashing the error code, shuts down the unit, and then starts a new high-heat cycle.

Steady Operation

During normal operation, if the high-heat pressure switch opens and doesn’t close again within 30 seconds, the 2-stage furnace control system shows Status Code 31.

If the switch remains open for more than 2 minutes, the control system continues to flash the error code, shuts down the furnace unit, and initiates a fresh high-heat cycle.

More useful resources: https://www.youtube.com/watch?v=tPCALBMcnZw

Status code troubleshooting manual