Quick answer
Code 31 on a Carrier furnace means the pressure switch did not close, or closed and reopened, so the control cannot confirm the vent is drawing. On two-stage models 31 is the high-heat pressure switch or relay. The furnace will not light. Clear snow, ice and debris from the outdoor intake and exhaust, cycle power once, and call if it returns.
On a Carrier furnace, code 31 means the pressure switch did not close or reopened. That is the single-stage wording; on two-stage models, 31 points specifically to the high-heat pressure switch or its relay, and the low-heat switch has its own code, 32. Either way the control has not been shown that the inducer fan is pulling air through the furnace and out the vent, so it refuses to open the gas valve. The status light flashes it as three short and one long.
What the pressure switch proves before the burners light
The pressure switch is a small diaphragm switch connected to the inducer housing by a rubber tube. When the inducer starts, it creates a slight vacuum; the diaphragm moves, the switch closes, and the control knows combustion gases have a path outside. No vacuum, no closed switch, no ignition. Code 31 covers both versions of the failure: the switch never closed at start-up, or it closed and then dropped out while the furnace was running.
Which model you have changes the reading, so check the code legend on the furnace door. If the legend and this page differ, the legend is right for your furnace.
Can a furnace run with code 31?
No, and it should not. A furnace that will not light because the switch is open is doing what it was built to do: without proof of draft, burning gas could spill combustion products into the house. Never jumper or bypass a pressure switch to get heat. A jumpered switch removes the one device that checks the vent before every cycle.
The safe step for code 31: clear the vent terminations outside
High-efficiency furnaces vent through plastic pipes, usually out a side wall. Go outside and find the intake and exhaust openings.
- Clear snow away from both openings and keep the ground below them shovelled. On 1970s–90s houses that were later converted to a high-efficiency furnace, the pipes often leave through the basement wall close to grade, where a drift can cover them overnight.
- Knock off ice by hand. Do not chip at the pipe with a tool.
- Look inside each opening for leaves, nests or a screen packed with lint or frost.
- Go back in and switch furnace power off and on once.
If the furnace lights and stays lit, the vent was the cause; the longer version is in what to do if a furnace vent is blocked by snow. Leave the tubing, the switch and the drain trap inside the cabinet alone.
Likely causes of code 31, most common first
- A restricted or blocked vent or intake: snow, ice, debris, or a vent pipe that has sagged and holds water.
- A plugged condensate drain or trap. Water backs up toward the inducer and the switch cannot pull in. In an unheated space the line can also freeze; see what to do if the furnace condensate line is frozen.
- A disconnected, cracked or obstructed pressure tube between the inducer and the switch.
- A weak inducer motor that spins but no longer makes enough vacuum.
- Excessive wind. Gusts against the termination can make the switch drop out; vents facing open exposure, such as lakefront lots on Lake Ontario or the Escarpment brow, are the usual candidates.
- A defective pressure switch. It does fail, but it gets replaced far more often than it is at fault.
How a technician diagnoses code 31, and repair costs
The test is a manometer teed into the pressure tube, reading the actual vacuum the inducer produces against the closing point printed on the switch. Enough vacuum with an open switch means a bad switch. Too little vacuum sends the technician to the drain, the vent run and the inducer. This is work inside a gas appliance, so it belongs to a licensed gas technician.
- Condensate drain cleaning or trap replacement: $150–$350.
- Pressure switch replacement: $250–$500.
- Draft inducer motor: $400–$900.
- Diagnostic visit: $90–$150, counted toward the repair.
If the terminations are clear and code 31 keeps coming back, book the diagnosis instead of buying a part. ZK Mechanical does furnace repair from Burlington through Oakville, Milton, Hamilton and Mississauga; the other codes are in the Carrier furnace error code guide. Call (647) 801-1252 or tell us what the furnace is doing.