Equipment error code

Carrier code 31

Pressure switch did not close (or did not open when expected) — the control monitored the inducer and pressure switch circuit and found the switch state out of sequence with the ignition chart.

Equipment: furnace
Brand: Carrier
Source checked: 2026-03-01

What this code means

Pressure switch did not close (or did not open when expected) — the control monitored the inducer and pressure switch circuit and found the switch state out of sequence with the ignition chart.

Source: Carrier 59MN7 Service Instructions — Fault Code Definitions (099-35-04PY-0A) · checked 2026-03-01

Which models show it

Carrier/Bryant condensing and mid-efficiency furnaces with induced draft — 59MN7, 59TN6, 59SC5A, 58MCA, 58MVP, and related 59SP series.

What is actually happening

The inducer may run continuously or the furnace may never advance to ignition because the board does not see the pressure switch change state. Heat never starts or stops early.

Is it safe to keep running?

A pressure fault usually prevents ignition, so the furnace will not heat. Running repeated reset cycles stresses the inducer motor without fixing venting.

Shut it down when

Stop if you hear the inducer laboring, smell exhaust in the utility room, or see water backing up in the condensate line on a 90%+ furnace.

What a homeowner can safely check

Inspect the small rubber hoses from the inducer to the pressure switch — disconnected hoses are a common find.
On high-efficiency units, clear the condensate trap — a full trap can prevent the switch from closing.
Check the exhaust and intake PVC outside for ice, nests, or snow blockage (keep 12+ inches clear).
Confirm the furnace intake and exhaust are not capped or swapped after service.

When to stop

Ice or debris you cannot safely clear from roof-line terminations.
Suspected flue corrosion or condensate leaking inside the cabinet.
Code 31 persists after hoses are reconnected and trap cleared.

Likely causes ranked

1. Blocked vent or intake (ice, debris, improper termination)
How a tech confirms: Measures draft and combustion air; inspects vent length and fittings against installation manual.
Typical repair: $175–$450 · Air Conditioner Repair cost guide
2. Failed pressure switch or stuck diaphragm
How a tech confirms: Applies calibrated vacuum to switch terminals; replaces switch if opens/closes out of spec.
Typical repair: $200–$425 · Air Conditioner Repair cost guide
3. Weak or failed inducer motor
How a tech confirms: Measures inducer amp draw and RPM; inspects wheel for cracks or debris.
Typical repair: $350–$850 · Furnace Replacement cost guide
4. Cracked condensate collector or blocked secondary heat exchanger
How a tech confirms: Pressure tests collector; inspects secondary for blockage on condensing models.
Typical repair: $400–$1,200 · Furnace Replacement cost guide

Is this the end of this unit?

Secondary heat exchanger or inducer failures on furnaces over 15 years often push replacement math. Compare repair quotes to a new high-efficiency furnace install.

How to clear the code

Clear vent blockages and condensate, reconnect hoses, then power cycle once. Allow the inducer to run through its pre-purge before expecting ignition.

Why clearing is not fixing it: The switch must prove draft before ignition. Clearing the code without restoring venting or switch function brings code 31 back immediately.

Which trade to call

Call a licensed hvac contractor who works on furnace equipment. Gas, refrigerant, and combustion faults are not general handyman jobs.

More codes from this brand

Symptom guide: Furnace Not Heating

FAQ

What does Carrier furnace code 31 mean?

Code 31 is a pressure switch fault — the control did not see the correct open/closed sequence from the draft proving switch during startup.

Can a clogged condensate line cause code 31?

On condensing furnaces, yes. Water in the collector can block draft proving. Clear the trap and hoses, then retry once.

Is code 31 dangerous?

It usually prevents ignition, which is safer than firing without draft. Do not bypass the pressure switch — that removes a critical safety check.

How much does fixing code 31 cost?

Hose reconnection or trap clearing during a service call is often $175–$250. Pressure switch or inducer replacement runs $200–$850 depending on parts.

Why does the inducer run but nothing else happens?

The board is waiting for pressure switch proof. Stuck switches, blocked vents, or weak inducers are the usual causes mapped to code 31.

Source

Source — Carrier 59MN7 Service Instructions — Fault Code Definitions (099-35-04PY-0A). Checked 2026-03-01.