Carrier Furnace Code 31: What It Means & How to Fix It
Compiled & reviewed by Nikolai Tsyrenov · Updated 2026-06-25 · confidence: medium
What does Carrier furnace code 31 mean?
Pressure switch or relay did not close, or reopened, when it should have. (Manual revisions print this as either the 'High-Heat' or 'Medium-Heat' pressure switch/relay — the underlying fault is the same.). Most often caused by Blocked, disconnected, kinked, or water-filled pressure-switch tubing.
What Carrier furnace code 31 means
Pressure switch or relay did not close, or reopened, when it should have. (Manual revisions print this as either the 'High-Heat' or 'Medium-Heat' pressure switch/relay — the underlying fault is the same.).
Common causes
Blocked, disconnected, kinked, or water-filled pressure-switch tubing
Blocked flue/vent or combustion-air intake, or excessive vent length
A weak or failing inducer (draft) motor not generating enough draft
A failed pressure switch or pressure-switch relay
Homeowner-safe steps to try
Check the outdoor vent/intake termination(s) and clear any obvious blockage — snow, ice, leaves, nests, or debris
Make sure nothing inside is leaning against or covering the venting near the furnace
Cycle power at the furnace switch to let the control retry
Do NOT open the furnace to adjust pressure switches or tubing — that is combustion-system work
When to call a licensed HVAC technician
If the basic checks below don't clear the code, the fault recurs, or anything involves the gas valve, burners, flame sensor or wiring, stop and book a licensed HVAC technician. Repairs to the combustion or gas system are not DIY.
⚠️ Safety first. Gas furnaces burn fuel and produce combustion gases. If you ever smell gas (a rotten-egg or sulfur odor), hear hissing, or your carbon-monoxide alarm sounds, do not touch light switches, thermostats, or the furnace — leave the home immediately and call your gas utility's emergency line or 911 from outside. Homeowners should limit themselves to simple, safe steps: setting the thermostat, checking and replacing the air filter, confirming the furnace switch and breaker are on, keeping supply registers and return grilles open, and clearing snow, ice, or debris from outdoor vent and intake terminations. Never bypass, jumper, or reset safety switches (limit, flame-rollout, or pressure switches), never open the gas valve or burner compartment, and never attempt combustion, electrical, or control-board repairs yourself. Error codes point toward a problem but are not a substitute for proper diagnosis — when in doubt, turn the system off and call a licensed HVAC technician. Keep working carbon-monoxide detectors on every level of your home.
When to call a pro: Call a licensed HVAC technician if the code persists after clearing the outdoor vent/intake — internal pressure-switch, tubing, inducer, or relay diagnosis is required. Note: the exact wording of code 31 differs by manual revision (some print 'High-Heat', others 'Medium-Heat' pressure switch/relay) — your specific model's manual or blower-door label is authoritative.
Carrier furnace code 31 FAQ
What does Carrier furnace code 31 mean?
Pressure switch or relay did not close, or reopened, when it should have. (Manual revisions print this as either the 'High-Heat' or 'Medium-Heat' pressure switch/relay — the underlying fault is the same.).
What causes Carrier furnace code 31?
Common causes: Blocked, disconnected, kinked, or water-filled pressure-switch tubing; Blocked flue/vent or combustion-air intake, or excessive vent length; A weak or failing inducer (draft) motor not generating enough draft; A failed pressure switch or pressure-switch relay.
Can I fix Carrier furnace code 31 myself?
You can safely try: Check the outdoor vent/intake termination(s) and clear any obvious blockage — snow, ice, leaves, nests, or debris; Make sure nothing inside is leaning against or covering the venting near the furnace; Cycle power at the furnace switch to let the control retry; Do NOT open the furnace to adjust pressure switches or tubing — that is combustion-system work. Call a licensed HVAC technician if the code persists after clearing the outdoor vent/intake — internal pressure-switch, tubing, inducer, or relay diagnosis is required. Note: the exact wording of code 31 differs by manual revision (some print 'High-Heat', others 'Medium-Heat' pressure switch/relay) — your specific model's manual or blower-door label is authoritative.
Sources: shareddocs.com · manualslib.com. General information for homeowners, not professional advice — confirm against your furnace's manual and a licensed technician.