How to Fix Carrier Furnace Error Code 31: Pressure Switch Did Not Close (Full Guide)

Carrier Furnace Error 31 indicates a “Pressure Switch Did Not Close” or “Pressure Switch Reopened” fault. In plain English, your furnace’s brain (the control board) is trying to start the combustion process but has detected that the venting system isn’t safely clearing out exhaust gases. As a safety precaution, the system locks itself out to prevent dangerous carbon monoxide from backing up into your living space.

Listen, I’ve been under the hood of these units for thirty years. If you’re waking up to a freezing house, hearing the inducer motor spin up only to shut down a minute later, or feeling cold air blowing from your vents, don’t panic. Error 31 is one of the most common codes we see, and nine times out of ten, it’s a maintenance issue rather than a total system failure. We can get this fixed, but you need to follow the trail of crumbs the machine is leaving for you.

Symptoms of a Pressure Switch Failure

When your Carrier furnace starts acting up with an Error 31, it doesn’t just stop quietly. It gives you physical cues. First, look at the small LED “peep hole” on the lower blower door. You’ll see a sequence: three short flashes, followed by one long flash. That’s your code 31.

Mechanically, you’ll notice the small “inducer” motor (the one that vents the gas) kicks on and whirs loudly. You might even hear the “click” of the igniter trying to glow, but the flames never arrive. After a few tries, the blower fan might kick on and stay on, circulating stone-cold air through your house. If you hear a “sloshing” sound near the unit, that’s a dead giveaway that water is trapped where it shouldn’t be, preventing that switch from closing.

The Complete Solution: Step-by-Step Fix

Step 1: Safety First and Power Down. Before you touch anything, flip the service switch on the side of the furnace to “Off” or pull the circuit breaker. We’re working around 120V electricity and moving parts; don’t be a hero. Use your 1/4″ nut driver or Phillips screwdriver to remove the upper cabinet door.

Step 2: Inspect the Vacuum Tubing. Look for the thin rubber hose connecting the pressure switch to the inducer motor. Pull it off and check for cracks, brittle ends, or water inside the tube. If there is water in the tube, you have a drainage issue (see Step 5). If the tube is cracked, snip off the bad end or replace the whole line for a few bucks.

Step 3: Clear the Collector Port. This is the “old pro” secret. Take a small piece of stiff wire or a paperclip and gently poke it into the small plastic nipple on the inducer housing where the hose was attached. Often, a tiny bit of white oxidation blocks this hole. Clearing it often restores the vacuum signal instantly. Warning: Do not shove the wire in too hard; you just want to clear the orifice.

Step 4: Check the Venting and Chimney. Go outside. Look at your PVC exhaust pipes. Are they covered in snow? Is there a wasp nest inside? Even a partial blockage can cause an Error 31. If you have a standard efficiency furnace with a metal chimney, check for sagging pipes that might be collecting water or bird debris.

Step 5: Inspect the Condensate Trap. Locate the plastic box (usually white or black) where the drain lines meet. If this is backed up with gunk, the furnace won’t vent. Remove the trap, flush it with warm water and a little bleach to kill the algae, and make sure the lines are clear. This is the most common cause of “intermittent” Error 31 codes.

Step 6: Electrical Testing with a Multimeter. Switch your meter to Ohms (Resistance). With the power off and wires disconnected from the switch, it should show “Open” or “OL”. Switch the power on and wait for the inducer motor to reach full speed. Now, carefully test for 24V across the switch. If the motor is running but the switch stays open (shows no continuity or no voltage drop), and you’ve cleared the ports, the switch’s internal diaphragm is likely shot and the part needs replacement.

Why is my Carrier showing Error 31?

In my experience, the pressure switch itself is rarely the villain; it’s usually the “messenger” telling you something else is wrong. The switch is a safety diaphragm that measures the “vacuum” created by the inducer motor. If it doesn’t feel enough suction, it stays open. Here is why that happens:

• Plugged Pressure Ports: Over years of operation, the small plastic port where the rubber hose connects to the inducer housing gets “crusted” with oxidation and sediment. This tiny bit of debris is enough to block the vacuum signal, making the switch think there’s no airflow.
• Condensate Backup: High-efficiency furnaces produce water (condensate). If your drain lines are clogged with “biological slime” or if the collector box is full of water, the inducer can’t pull a vacuum. It’s like trying to breathe through a straw that’s underwater.
• Obstructions in the Flue: I’ve pulled everything from bird nests to dead squirrels and even ice dams out of furnace vents. If the exhaust can’t get out, the pressure switch won’t close. This is a common issue after a heavy snowstorm or in the fall.
• Ruptured Diaphragm: Inside that round pressure switch is a rubber disc. Over time, heat and age cause that rubber to dry out and crack. If it develops even a microscopic pinhole, it can’t hold the vacuum necessary to trigger the electrical contact.

How to Prevent Error 31

Once you get your heat back, you don’t want to be doing this again in three weeks. Here is how you keep that code away for good. First, annual condensate flushing is non-negotiable. Every autumn, pour a cup of warm water and a teaspoon of bleach through your condensate lines to prevent the “slime” that causes backups. This is the number one preventative measure for high-efficiency Carriers.

Second, check your vent screens. If you don’t have screens on your exterior PVC pipes, get some. They prevent rodents and birds from making a home in your furnace during the off-season. Finally, ensure your air filters are changed every 90 days. While Error 31 is usually about the exhaust side, poor return airflow can cause the entire cabinet to run hot, which stresses the plastic components and rubber diaphragms of your safety switches.

Frequently Asked Questions

Q: Can I just bypass the pressure switch to get heat tonight?
A: Absolutely not. I’ve seen what happens when people “jumper” these switches. If the switch isn’t closing, it’s because the furnace isn’t venting. Bypassing it can lead to carbon monoxide poisoning or a fire. Never, under any circumstances, bypass a safety switch in a furnace.

Q: My inducer motor is making a buzzing sound but not spinning. Is that Error 31?
A: Yes, if the motor doesn’t spin, it can’t create the vacuum to close the switch. If the motor is buzzing, the capacitor might be bad, or the motor bearings have seized. In this case, the switch is doing its job by staying open—it’s the motor that needs your attention.

Q: The code only happens when it’s extremely cold outside. Why?
A: This is usually “Ice Hoar.” Moisture in the exhaust freezes at the tip of the vent pipe outside, slowly narrowing the opening until the pressure switch can no longer “breathe.” Clear the ice from the exterior pipe and the code should clear.