The short answer
Code 31 on a Carrier furnace in Northern Virginia means the pressure switch didn’t close, or it closed and then opened again during the heating cycle. The switch is a safety. It only closes once the inducer motor, the small fan on top, has pulled enough draft through the vent pipe. Carrier’s checklist for a 31 comes down to four things you can see: a plugged condensate drain, a blocked or sagging vent, a kinked or water-filled pressure tube, and an inducer motor that’s wearing out.
What you’ll notice
- The inducer starts and hums, but the burners never light.
- The furnace tries, quits, waits and tries again. When the switch stays open more than 5 minutes, Carrier’s board shuts the inducer off for 15 minutes, then retries.
- The light behind the window in the lower door flashes 3 short, then 1 long.
- It runs fine for days and throws 31 on a cold night, when the furnace runs longest and makes the most condensate water.
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How to read the code
Look through the small window in the lower door. Short flashes are the first digit and long flashes are the second, so 31 is 3 short and 1 long, then a pause before it repeats. A steady light is normal, and rapid flashing means the line voltage polarity is reversed. On older boards with a red light, cutting the power erases the code, so count it before you flip the switch. Newer Carrier boards keep the last 7 faults through a power cut and print a code with a third digit, like 31.6, on a small display. Our full list of Carrier furnace codes has every one.
What causes code 31
These are the checks Carrier lists for a pressure switch that won’t close, with the ones a homeowner can look at first.
Swipe to see all columns →
Swipe to see all columns →
| Cause | What it looks like | Can you check it? |
|---|---|---|
| Condensate drain or trap plugged | Gurgling, water around the furnace, a condensate pump full and not running | Yes: look at the hose and the pump |
| Vent pipe blocked or holding water | Leaves, a nest, snow or ice at the pipe ends outside; a sagging run that holds water | Yes: look at the pipe ends outside |
| Pressure switch tubing | The small rubber hose to the switch is kinked, cracked, off its port or full of water | Look, and push it back on if it slipped off |
| Inducer motor | Louder than it used to be, slow to spin up, a grinding or rattling sound | No: we measure it |
| Wind | 31 only on gusty days, when wind blows across the vent ends | No: the vent ends may need a change |
| Gas pressure, combustion air, vent size | Mostly on a new install, or after a remodel closed in the furnace room | No: these get measured |
On a two-stage Carrier, 31 points at the high-heat pressure switch and 32 at the low-heat switch, and the list of causes is the same. Newer boards split it further: 31.6 means the high-heat switch didn’t close within 75 seconds of the furnace stepping up from low heat to high.
What you can safely check
- Go outside and look at the white PVC pipe ends. Clear leaves, a nest, snow or ice from around them, and don’t push anything into the pipe.
- Look at the condensate hose and pump beside the furnace. A full pump that isn’t running, or a kinked hose, is your lead. Our condensate drain guide walks through it.
- Find the small rubber tube on the pressure switch. If it slipped off, push it back on snugly. If it’s cracked or holds water, leave it for us.
- Count the code, then turn the furnace switch off for 30 seconds and back on and let it try once.
What not to do
Never jump or bypass a safety switch to get heat back. Each one is there to stop the furnace from burning gas when something is wrong, and the wrong thing is often exhaust, which carries carbon monoxide. If your carbon monoxide alarm goes off, get everyone outside and read our CO alarm guide from there. And don’t buy a new pressure switch on a hunch. A new switch on a furnace with a plugged drain trips the same way the old one did.
What a technician does
We read the fault history, then put a manometer on the switch and compare the draft the inducer actually pulls with the number printed on the switch. That one reading tells us whether the problem is the drain, the vent, the tube, the inducer or the switch. Then we clear the trap and drain, check the vent run for sags that hold water, and test the inducer motor.
What it costs to have us look
A repair visit starts with a flat $99 diagnostic, and you get the price of the fix before we touch anything. Our Carrier repair page lists what the common repairs cost. Our heating check is $67 per system, through December 31: furnace, heat pump or boiler.
Related: furnace condensate drain clogs, furnace short cycling, furnace won’t turn on and our Carrier repair page.
Every code meaning here comes from Carrier’s own documents: 58SC0A installation instructions, 58TP0B manual and 59MN7C manual.
Call or book
Call (703) 997-9757 and tell us the code you counted, or book online from goswiftpro.com/cookiesoffer and a box of warm cookies comes with the visit. Rated 5.0 stars across 1,138 reviews on Google, Angi, Yelp, the BBB and Facebook, Virginia Class A license 2705192276.
Questions people ask
What does code 31 mean on a Carrier furnace?
The pressure switch didn’t close, or it reopened during the heating cycle. The furnace won’t light until the inducer pulls enough draft through the vent to close it.
Can I reset a Carrier furnace showing code 31?
Count the code first, since older boards erase it, then turn the power off for 30 seconds and back on to let it try once. If 31 comes back, something is still blocking the draft: the drain, the vent, the tube or the inducer.
Is it safe to bypass the pressure switch?
No. The switch keeps the furnace from burning gas when the exhaust can’t leave the house, and that exhaust carries carbon monoxide.
What’s the difference between Carrier code 31 and code 32?
On a two-stage Carrier furnace, 31 is the high-heat pressure switch and 32 is the low-heat pressure switch. The causes are the same list.
How much does it cost to fix code 31?
The visit starts with a flat $99 diagnostic, and you get the price of the repair before any work starts. Our Carrier repair page lists the common repairs.
