Carrier Furnace 31 Error Code: Meaning, Causes & Fix
Pressure switch did not close (or opened) during operation — the inducer couldn't prove proper venting draft.
Before Carrier furnaces light, the inducer fan runs and the pressure switch must close to prove the vent is drafting. Code 31 means that proof failed: the switch never closed (or dropped out) when it should. The chain is inducer → vent path → pressure switch tubing → the switch itself. A blocked vent (snow, nests, ice at the termination) is both common and the safety-relevant cause: the switch exists to stop the furnace firing into a blocked flue. Code 31 is the furnace refusing to fire on faith. The sequence it guards is absolute: inducer first, draft proven by the pressure switch, and only then ignition — so a 31 is never about flame, gas or heat; it is about air moving through a pipe. That is why its cause list reads like plumbing — vent sizing, sags, traps, drains, frost, wind — and why its most misread symptom is the switch chattering at start-up, which sounds electrical but is usually water moving in the vent. Read 31 as 'I cannot yet prove my exhaust goes somewhere safe': restore the air path, measure before replacing, and treat the switch as the witness, not the culprit, until the manometer says otherwise.
What causes the 31 code on a Carrier furnace?
Blocked or restricted vent/intake
Snow/ice at the PVC termination, nests, a sagging vent run holding condensate.
Condensate problems
A clogged condensate trap/drain backs water into the inducer/vent area and strangles draft — condensing furnaces' classic 31.
Cracked or water-filled pressure switch tubing
The little hose from inducer to switch must be clear and dry inside.
Weak inducer motor
Bearings tire; draft falls below the switch's threshold.
Pressure switch failure
The switch itself — last, as always with proving switches.
The switch that chatters: a sagging vent or a clogged trap, by Carrier's own step
Carrier's variable-speed flow asks a question most lists never reach: does the high-heat pressure switch make, then break, several times at the start of the cycle before faulting out? A yes does not condemn the switch — the guide answers with a sag in the vent pipe, a clogged drain trap or line, or improperly plumbed drainage tubing within the furnace. Water pooled in a low vent section surges as the inducer runs, draft oscillates, and the switch honestly reports each oscillation. A 31 arriving with gurgling, or minutes into a cycle, is often this water story, not a failed part.
Generation split: on variable-speed Carrier furnaces, 31 is the HIGH-heat switch
Read 31 with your model in mind. On Carrier's mainstream charts (58MCB, 59SC2 and similar), 31 covers the pressure-switch family — a low- or high-stage switch did not close or reopened. On the variable-speed two-stage generation (58MVP, 58CVA, 58MVC), Carrier splits the pair: 32 reports the low-heat switch and 31 the high-heat switch or relay — including stage-transition cases where it opens mid-cycle. Same two digits, a narrower suspect — which is why your model's door chart outranks any general list.
Frost at the inlet, and vent pipes reversed
Two of Carrier's documented 31 findings live outside the furnace: frost build-up on the combustion-air inlet, and vent and combustion-air pipes reversed — or moist air entering the combustion-air pipe — after installation or roof work. Both starve or confuse the draft the switch proves. A 31 debuting in a cold snap with a frosted intake, or right after vent work, should be read at the pipes before it is read at the switch.
A miswired gas valve wearing a pressure code
Carrier's 31 section opens with a wiring warning: a miswired gas valve can cause this problem, and the guide specifies the landing — the BLUE wire to the gas valve's M terminal, the GREEN/YELLOW wire to its C terminal. After any gas valve replacement, a fresh 31 may be an electrical mix-up, not a draft failure. Recent-work correlation is diagnostic gold on this code.
Checks to run before you replace anything
- SAFETY: never bypass a pressure switch or run the furnace with it jumpered — it proves your exhaust has somewhere to go.
- Check the outdoor vent/intake terminations for snow, ice, nests or debris; clear them.
- Check the condensate trap and drain line for blockage (a trap you can see through should show free water movement); clear and flush.
- Inspect the pressure switch tubing for cracks, sags holding water, or a loose fit (power off).
- Watch the start of a cycle and time the failure: on the variable-speed control, a high-heat switch that fails to make shortly after the low-heat switch made (Carrier's windows are tens of seconds — about 45 seconds on the 58MVP, 30–45 on the 58CVA) produces 31, a shutdown and a restart. A furnace repeating that rhythm is showing draft that never quite arrives: restriction or inducer strength, not electrics.
- Listen for the chatter signature: the switch making and breaking at start-up, heard as the inducer surging with gurgling, is Carrier's sag-or-trap branch. Check the condensate trap and drain first, and sight along the vent run for a belly holding water, before pricing a switch.
- Use the mainstream timing as a clock on single-chart boards: if the switch stays open longer than five minutes, the inducer shuts off for 15 minutes before retrying. Finding the furnace silent mid-31 is that pause, not a second fault — wait it out once, and spend it clearing the terminations and trap.
- Check the intake for frost and both terminations for snow after cold, windy weather, and ask whether any vent work was recently done (Carrier lists reversed vent and combustion-air pipes as a 31 cause). Weather-locked and work-locked 31s are both read outside the cabinet.
How to fix the 31 error code
- Clear vent/intake obstructions; correct a sagging vent run so condensate drains back as designed.
- Clean the condensate trap and drain; replace perished tubing.
- Replace the inducer motor if it can't make draft with a clear path (technician confirms with a manometer — draft is measured, not guessed).
- Replace the pressure switch only when measured draft exceeds its rating and it still won't close.
- Clear the water path completely: flush the condensate trap and drain line, correct any vent sag so condensate runs back to the furnace as designed, and re-plumb drainage tubing inside the cabinet if it was routed wrongly. Then demand a full cycle as proof, because the chatter branch only fails under running draft.
- Let a manometer arbitrate switch-versus-draft: Carrier's flow connects a slope manometer across the heat exchangers and replaces the pressure switch assembly only when the measured pressure drop meets the specified value (1.90 in. w.c. on the 58MVP) while the switch still will not make. Draft below specification behind a clear vent sends the job to the inducer — check its wheel and rotation before condemning the motor.
- Correct reversed or frost-prone piping at the source: put reversed vent and combustion-air pipes back where they belong, clear inlet frost, and address why moist air is entering the combustion-air pipe. A 31 cured by pipe correction needs no parts — and a switch replaced instead inherits the same wet, starved draft and codes again within days.
The part that usually fails: Usually no part — vent/condensate obstruction (inducer motor next)
A manometer reading of the draft vs the switch's printed rating (inches of water column) is the definitive test — HVAC technicians carry it; guessing condemns good switches.
How to reset / clear the 31 code
31 clears automatically when the switch closes on a normal start sequence — after the obstruction is cleared, cycle the thermostat off/on to restart. Persistent 31 after clear vents/trap = measurement time (technician).
Carrier fault codes are stored by the control board and shown as two-digit codes via the status LED (short flash = first digit, long flash = second digit, with a pause between). Most codes clear automatically once the fault condition is gone and a new call for heat succeeds; a hard lockout (such as code 13 or 14) needs the fault fixed, then power cycled at the furnace switch or breaker for a few seconds, and in some lockouts the board waits up to an hour or requires several successful cycles before normal operation resumes. Clearing a lockout without fixing its cause just restarts the countdown to the next lockout.
Why does the 31 code keep coming back?
31 returning in deep cold snaps points at termination icing or a condensate line that freezes; returning after rain points at vent water ingress/sag. The weather correlation is the diagnosis. A returning 31 keeps a calendar as much as a fault. Returns in deep cold point to termination frost or a freezing condensate run; returns after rain point to vent water ingress or a sag that fills only when driven rain gets in; returns that shorten their interval over a season point to a trap slowly clogging or an inducer slowly weakening. The return that should end DIY is a 31 with clear vents, a clear trap and sound tubing — from there the diagnosis is a manometer reading against the switch's printed rating, a technician's visit. What never belongs in the plan is defeating the switch: every return of 31 is the furnace declining to fire into an unproven vent, and that refusal is the safety system working.
How to stop 31 coming back
- Blocked or restricted vent/intake: Snow/ice at the PVC termination, nests, a sagging vent run holding condensate.
- Condensate problems: A clogged condensate trap/drain backs water into the inducer/vent area and strangles draft — condensing furnaces' classic 31.
- Cracked or water-filled pressure switch tubing: The little hose from inducer to switch must be clear and dry inside.
Retrieving stored fault codes on Carrier furnaces
Carrier boards include a component test mode (a test pin/shorting jumper on the board, per the installation manual) that runs the inducer, blower and ignition sequence on demand, and the board keeps the last faults in memory readable as LED codes. The full fault chart is printed on the blower-door label of every unit — photograph it before relying on any online chart, because code assignments differ slightly between board generations.
Other Carrier furnace error codes
| Code | Meaning | First fix |
|---|---|---|
| 11 | No previous code — the control's stored fault history is empty; this is not a fault | No repair needed: run a heating or cooling cycle to check the system; a furnace flashing 11 simply has nothing stored |
| 12 | Blower on after power-up — not a fault: the control runs the blower for ~90 seconds at start-up when a thermostat call was present during a power interruption (a status/safety behaviour). | Nothing, if heating follows. If the furnace stalls at 12 behaviour repeatedly, the control or thermostat-call circuit needs a technician's look. |
| 13 | Limit circuit lockout — the control locked the furnace out because a limit or flame rollout switch in the limit circuit stayed open longer than 3 minutes, or limit trips came in succession (10 successive trips during high-heat; the variable-speed generation also locks out after 3 successive trips during low-heat). The control auto-resets after 3 hours. | Restore airflow: new filter, open registers, clean the blower wheel (and have the indoor coil cleaned if it's matted — HVAC technician work). |
| 14 | Ignition lockout — the furnace tried to light three times and failed, so the control locked out (typically retrying after ~1 hour). | Clean the flame sensor (power off; remove it and polish the rod gently with a fine abrasive pad/scotch-brite — no sandpaper grit damage, just the oxide) if light-then-die is the pattern. |
| 15 | Blower motor lockout — the blower failed to reach 250 RPM or to communicate within 30 seconds of turn-on, in two successive heating cycles | The control auto-resets after 3 hours; diagnose as code 41 — licensed HVAC technician work |
| 21 | Gas heating lockout — Carrier's service label words it: 'STATUS CODE 21 — GAS HEATING LOCKOUT. Control will NOT auto reset. Check for: mis-wired gas valve; defective control (valve relay).' The control has shut down gas heating and, uniquely among Carrier's common lockouts, it will not clear itself with time. | The one owner-level step Carrier sanctions: turn power off, wait 5 minutes, restore power and set the thermostat to call for heat once. If 21 does not return and the furnace completes a normal heating cycle, the lockout is cleared. If it returns, stop — the remaining steps are licensed-technician work. |
| 22 | Abnormal flame-proving signal — flame is proved while the gas valve is de-energised; the inducer runs until the fault clears | Turn the furnace off and do not run it on this code; a leaking gas valve is licensed HVAC technician work |
| 23 | Pressure switch did not open — a pressure switch is still closed (made) when a call for heat begins | Check the pressure tubing for blockage and clear it; switch testing is licensed HVAC technician work — never jumper the switch |
| 24 | Secondary voltage fuse open — the 3-amp fuse on the control board has blown (24V circuit short/overload). | Repair the short (tape/replace damaged wire sections, correct rub points with a grommet). |
| 25 | Invalid model selection or setup error — the model plug is missing or incorrect, or a setup switch is wrongly set | Four flashes at power-up only: the control defaults to its stored model and still runs; correction is licensed technician work |
| 32 | Low heat pressure switch did not close / opened (on two-stage Carrier furnaces: the low-fire pressure switch proof failed). | Clear restrictions; clean condensate path; replace bad tubing. |
| 33 | Limit circuit fault — a limit, flame rollout, draft safeguard or blocked-vent switch (as fitted) in the limit circuit is open right now, but the control hasn't escalated to lockout (13) yet. | Restore airflow (filter, registers, wheel/coil cleaning). |
| 34 | Ignition proving failure — flame was not sensed in the trial for ignition; the control tries 3 more times, then lockout code 14 | Confirm gas and the manual valve are on; the rest is licensed HVAC technician work |
| 35 | Gas valve fault (modulating models) — the modulating gas valve failed to respond to a control command, or power to its electronics was interrupted | Gas valve and wiring diagnosis is licensed HVAC technician work |
See the full list on the Carrier Furnace error codes hub.
Codes 31 gets confused with
32 — Low heat pressure switch did not close / opened (on two-stage Carrier furnaces: the low-fire pressure switch proof failed). Two-stage Carrier furnaces prove draft twice — once per firing rate, with separate pressure switches.
43 — Low-heat pressure switch open while the high-heat pressure switch is closed — the stage switches disagree. That is the mainstream Carrier chart's wording (58CVA service label: 'LOW-HEAT PRESSURE SWITCH OPEN WHILE HIGH-HEAT PRESSURE SWITCH IS CLOSED'). On the older 58MVP variable-speed generation the same code is titled PRESSURE SWITCH CALIBRATION FAULT — see the honesty note in the what and FAQ: both titles are Carrier's own, on different generations. A two-stage Carrier furnace proves its vent with two pressure switches set at different drafts: the low-heat switch makes at the gentle draft of low fire, and the high-heat switch needs the stronger draft of high fire before it will make.
33 — Limit circuit fault — a limit, flame rollout, draft safeguard or blocked-vent switch (as fitted) in the limit circuit is open right now, but the control hasn't escalated to lockout (13) yet. 33 is 13's earlier chapter, and Carrier's service instructions define it by the circuit, not just by overheating: a limit OR flame rollout switch is open.
21 — Gas heating lockout — Carrier's service label words it: 'STATUS CODE 21 — GAS HEATING LOCKOUT. Control will NOT auto reset. Check for: mis-wired gas valve; defective control (valve relay).' The control has shut down gas heating and, uniquely among Carrier's common lockouts, it will not clear itself with time. Code 21 is the Carrier control declaring that it cannot trust its own gas valve output.
Frequently asked questions
Can snow really shut down a furnace?
Yes — a drifted-over PVC intake/vent is one of the most common winter 31s. Clear around the terminations after storms (keep the area on your snow-clearing list).
Why does my furnace start, then stop, with 31?
The switch closed marginally then dropped out as conditions changed — classic for a partially blocked vent, water in the tubing, or a weakening inducer.
My Carrier furnace flashes 3 short 1 long — what is error code 31?
That pattern IS code 31: three short flashes give the first digit (3) and one long flash the second (1). Code 31 means a pressure switch did not close or reopened — the inducer could not prove venting draft. On variable-speed two-stage Carrier models, 31 names the high-heat pressure switch specifically; on the mainstream chart it covers the pressure-switch family. Carrier's checks run from vent sizing and restrictions through condensate, tubing and the inducer to the switch itself.
I read my Carrier furnace code as 1 long 3 short — is that code 31 or a different Carrier furnace code?
It is 31, read backwards. Carrier codes are always read short flashes first, then long flashes — first digit from the short count, second from the long count — and there is no 1-long-3-short code in Carrier's chart. A pattern remembered that way is 31 (3 short, 1 long) picked up mid-cycle. Watch two full rounds from the pause, or video the LED through the sight glass, and read it in Carrier's order before diagnosing.
How to fix error code 31 on a Carrier furnace?
Work Carrier's order — prove the path before the part. Clear the outdoor vent and intake terminations, flush the condensate trap and drain, check for a sagging vent section holding water, and inspect the pressure-switch tubing for disconnection, obstruction or water. If the path is clear, the remaining checks are measurement work: draft against the switch's rating with a manometer, inducer condition, and only then the switch. Never jumper the pressure switch to get heat — it is the proof your exhaust has somewhere to go.
Why does my Carrier furnace show code 31 only in very cold or windy weather?
Because both attack the draft margin the switch proves. Cold snaps frost the combustion-air inlet (one of Carrier's own listed findings for this code) and freeze condensate lines; wind pressure at the termination works against the inducer, and Carrier lists excessive wind on the 31 check list for exactly this reason. A weather-correlated 31 is a termination-and-venting diagnosis: keep terminations clear and, if it persists, have the vent sizing checked rather than replacing the switch on spec.
Related guides
Facts on this page were checked against manufacturer documentation: mans.io · mans.io · www.shareddocs.com · mans.io · www.manualshelf.com · manualsdump.com