Code 31 means your pressure switch is stuck open and preventing ignition. In most service calls, this is caused by a blocked vent pipe, a clogged condensate drain, or a failing inducer motor rather than a faulty switch.
If you ignore code 31, you've got no heat and a furnace that's just cycling on and off trying to restart itself. That won't fix anything on its own. Worst case, you've got frozen pipes in a few hours if it's really cold out. I've been on plenty of these calls where it turned out to be a wasp nest in the exhaust pipe or a condensate trap full of black slime. Both of those are free fixes if you catch them early.
Here's what I tell people when I pull up on a code 31 call. Nine times out of ten, nobody's touching the actual pressure switch. That thing's probably fine. What isn't fine is the vent pipe that's half-blocked, or the condensate trap that's been slowly filling with gunk all season. Carrier's high-efficiency furnaces run wet, meaning they generate a ton of condensate, so drainage problems are super common on these.
Most Likely Causes
Based on aggregated repair data, here is the probability breakdown for this error code:
Blocked Vents or Drains50%
Inducer Motor Failure25%
Faulty Pressure Switch15%
Tubing or Wiring Issues10%
Symptoms You May Notice
The inducer fan kicks on and runs for about 30 seconds, then everything shuts down and the status light starts flashing that 3-blink then 1-blink pattern.
You hear a wet, gurgling sound from inside the furnace cabinet every time it tries to start up.
Furnace attempts to cycle on 3 times in a row, each attempt a little shorter than the last, then goes completely silent and locks out.
No heat at all even though the thermostat is calling for it and you can hear something inside the furnace trying to run.
You look outside and the PVC exhaust pipe has frost or ice built up around the opening.
Can you reset a Carrier furnace to clear the 31 code?
Flip the power switch on the side of the furnace to OFF and wait a full 30 seconds, then flip it back on. No power switch? Kill it at the circuit breaker. The board needs those 30 seconds to fully clear the lockout. Then bump your thermostat a few degrees above room temp and watch the startup sequence. Inducer runs, switch closes, igniter clicks. If the code comes right back, the underlying cause isn't fixed yet.
Tools Required for Diagnosis
Phillips #2 screwdriver1/4 inch nut driverMultimeter (set to Ohms or continuity)Needle nose pliersThin wire or straightened paperclip (for clearing vacuum ports)Wet/dry shop vacuum (for clearing condensate drain lines)FlashlightBucket or old towels (condensate will spill when you pull the trap)
Diagnostic Checklist
Follow these steps in order. We start with the easiest external fixes before opening up the machine.
ComponentComponent Under Test
Expected Range0–1 ohms
ConditionIf Open (OL) or infinite, replace component.
Frequently Asked Questions
Can I bypass a Carrier pressure switch to get heat temporarily?
Don't do it. I know it's tempting when it's 2am and 10 degrees outside, but that pressure switch is what tells the board that combustion gases are actually going somewhere safe. If you jumper it and the vent's blocked, you're running a furnace that's pumping carbon monoxide into your house. And CO doesn't smell like anything. It's just not worth it. The vent pipe check takes five minutes. Do that first.
Why does my furnace run for 30 seconds and then show code 31?
That's the normal startup sequence on these Carrier boards. It fires up the inducer and waits up to 30 seconds for the pressure switch to close. If it doesn't get that signal in time, it shuts everything down and logs the 31. So the inducer ran, it just didn't build enough pressure. Start with the vent pipes outside. That's the most common cause and it takes two minutes to check.
Does a dirty furnace filter cause code 31?
Generally no. A dirty filter restricts the blower side and usually triggers code 33, which is the limit switch tripping from overheating. Code 31 is all about the combustion air and exhaust side. Completely different circuit. That said, change your filter while you're in there anyway. If it looks gray and dusty, it's been in way too long.
How much does it cost to fix a Carrier code 31?
If it's a blocked vent or a clogged drain, the cost is zero if you do it yourself. Pressure switch replacement is 30 to 60 bucks in parts and about 150 to 300 total if you call a tech. A full inducer motor replacement is where it gets expensive, budget 600 to 900 dollars for that. Honestly, most code 31s I see don't need any parts at all. It's usually just the vent or the drain.
Why is there water in the pressure switch tubing?
Usually one of two things. Either the furnace isn't pitched correctly so condensate runs toward the inducer instead of toward the drain, or your condensate trap is clogged and water's backing up through the system. Fix the trap and drain first. If water keeps showing up in the tube after that, check the furnace pitch. There should be a very slight slope toward the drain side. A shim under one side of the unit can fix it.
How do I know if it's the pressure switch or the inducer motor causing code 31?
Here's the quick test I use on every one of these calls. Disconnect the vacuum hose from the pressure switch. Start the furnace and when the inducer kicks on, put your finger over the open end of the hose. You should feel pretty strong suction. If you feel good suction but you're still getting code 31, the switch is bad. Barely any suction or nothing at all means the inducer is weak or there's a vent blockage. That one test takes 30 seconds and tells you exactly which direction to go.
Models Known to Experience 31 Errors
This repair applies to most Carrier furnaces with this error code. Common model numbers include: