If your Carrier furnace is flashing a 31 on the LED behind the blower door, it's telling you the pressure switch did not close — or it closed and then reopened partway through the cycle. That switch is a safety: it confirms the inducer (draft) motor is actually pulling combustion gases out through the vent before the board will let the burners light. When it doesn't confirm, the furnace won't fire. On Carrier, Bryant, and Payne furnaces (they share the same control board), code 31 is a common venting-side code.
Reading the code: Carrier boards flash a 2-digit code — short flashes count the tens, long flashes count the ones. Three short + one long = 31. Code meanings can vary by control-board generation, so always confirm against the diagnostic label printed inside your furnace's blower compartment.
What code 31 means
Code 31 is a pressure-switch fault: the switch didn't close on a call for heat, or it opened during the cycle. The board watches that switch to prove the inducer is drawing a proper draft. If the draft is weak, blocked, or the switch never sees the vacuum it expects, the furnace refuses to ignite so unburned gas and combustion products can't build up. It's a safety interlock doing its job.
It sits in a family of related Carrier codes:
| Code | Meaning | Typical cause |
|---|---|---|
| 31 | Pressure switch didn't close, or reopened during the cycle. This page. | Blocked intake/exhaust vent, clogged condensate trap, weak inducer, disconnected/cracked switch hose, blocked flue |
| 33 | Limit circuit fault (a soft over-temp trip) | Dirty filter, restricted airflow, weak blower |
| 13 | Limit circuit lockout | An ignored limit/rollout that stayed open too long |
| 34 | Ignition proving fault (flame not sensed) | Dirty flame sensor, low gas, weak igniter |
The takeaway: 31 is about venting and draft, not airflow across the heat exchanger (that's 33). Fix the draft path and it usually clears.
Common causes
When a Carrier throws 31, here's what's behind it, most common first:
- A blocked intake or exhaust vent. On high-efficiency furnaces that vent through PVC pipes outside, a bird nest, snow, ice, leaves, or insect debris in the termination is a leading cause. The inducer can't pull a proper draft.
- A clogged condensate trap or drain. High-efficiency (condensing) furnaces drain water; when that trap clogs, it can block the pressure switch from closing.
- A weak or failing inducer (draft) motor. If the inducer can't spin up to speed, it never creates the vacuum the switch needs.
- A disconnected, cracked, or blocked pressure-switch hose. The little rubber tube that senses draft can pop off, crack, or fill with condensate.
- A blocked flue or restricted venting. Anything that chokes the exhaust path.
The usual fix
Our tech confirms why the switch isn't closing rather than just replacing it: inspect the intake/exhaust terminations outside for blockage, check and clear the condensate trap and drain, measure the inducer motor and the draft it produces, inspect the pressure-switch hose for cracks or water, and test the switch itself against spec. Many code-31 calls end with a cleared vent or condensate trap; when the inducer motor or the pressure switch has genuinely failed, we replace it and confirm the furnace runs a full heat cycle without dropping out.
DIY vs. calling a pro
Safe to try yourself first (free):
- Check the outdoor intake and exhaust terminations. On high-efficiency furnaces, walk outside and look at the PVC pipe ends for a bird nest, snow, ice, or debris — clear anything blocking them.
- Look at the condensate drain for standing water or an obvious clog, and clear the trap if you're comfortable doing so.
- Power-cycle at the switch or breaker to clear the code after removing a blockage.
Call a pro when:
- Code 31 comes right back after you've cleared the vent and drain — the inducer, switch, or hose needs testing.
- The inducer motor is noisy, weak, or silent, or you find a cracked/disconnected hose you're not comfortable servicing.
- You have no heat and the furnace won't ignite at all.
Gas, line-voltage electrical, and any refrigerant or compressor work belong with a licensed technician (EPA 608 certified for refrigerant handling) — the inducer, gas valve, and combustion path involve line voltage and combustion safety, so a persistent 31 is worth a tech's eyes.
What it costs in the Bay Area
Many code-31 fixes are inexpensive because the cause is a blocked vent or a clogged condensate trap — often just cleared, no part. When a part has failed, the inducer (draft) motor is a distinct repair quoted by model, and a control board runs about $200–$600 in our pricing. We charge a diagnostic fee that's credited toward the repair when you approve the work, and you always get a firm, upfront price first.
Book Carrier furnace service
Triton services Carrier, Bryant, and Payne furnaces — plus every major brand — across the SF Bay Area & Peninsula, with same-week and often same-day appointments. If your Carrier is stuck on code 31, we'll trace the draft path and get your heat back.
Call (888) 993-5699 or book online for same-day furnace service.
Related pages
- HVAC Repair in the SF Bay Area — our full heating & cooling hub
- Carrier Furnace Code 33 (Limit Circuit Fault) — the airflow/limit code
- Furnace Not Heating / No Heat — the symptom behind a no-ignition code
- Carrier / Bryant HVAC Repair — brand service hub
- Furnace Repair in Burlingame — same-day service in your city
Frequently asked questions
What does code 31 mean on my Carrier furnace? Code 31 is a pressure-switch fault: the switch did not close on a call for heat, or it opened during the cycle. That switch proves the inducer is drawing combustion gases safely out through the vent before the burners light. When it does not confirm, the furnace refuses to ignite. Carrier, Bryant, and Payne furnaces share this code.
How do I read Carrier code 31? Carrier boards flash a 2-digit code where short flashes count the tens and long flashes count the ones. Three short flashes plus one long flash equals 31. Confirm the count against the diagnostic label inside the blower compartment, since meanings can vary by control-board generation.
What causes a Carrier pressure switch fault? The most common cause is a blocked intake or exhaust vent, especially a bird nest, snow, or debris in the outdoor PVC termination on high-efficiency furnaces. Other causes are a clogged condensate trap, a weak or failing inducer motor, a disconnected or cracked pressure-switch hose, or a blocked flue.
Can I fix a Carrier code 31 myself? You can safely check the outdoor intake and exhaust terminations for a blockage and clear them, and look at the condensate drain for a clog. Then power-cycle the furnace. If code 31 comes right back, the inducer motor, pressure switch, or hose needs testing by a technician.
Is Carrier code 31 the same as code 33? No. Code 31 is a venting and draft problem: the pressure switch did not close. Code 33 is a limit-circuit fault from restricted airflow across the heat exchanger, usually a dirty filter. They are different systems, so the fixes are different, though both stop the furnace.
How much does it cost to fix a Carrier code 31 in the Bay Area? Many 31s are cheap because the cause is a blocked vent or a clogged condensate trap, often just cleared with no part. When a part has failed, the inducer draft motor is a distinct repair quoted by model, and a control board runs about 200 to 600 dollars in our pricing. We credit the diagnostic fee toward the repair and quote a firm price first.