The pressure switch is the most misunderstood part on a Carrier furnace. Homeowners see code 31 and assume the switch is broken; technicians who throw parts replace it and the fault comes right back. The switch is rarely the problem. It is a diaphragm that closes a set of contacts when the draft inducer creates enough negative pressure, and it exists to prove the furnace is venting safely before any gas burns. Code 31 is that proof failing. This page is the Carrier-specific companion to our general furnace repair service and our Carrier furnace repair page.
How code 31 reads — conventional vs. Infinity
The same number surfaces two different ways depending on your system, and confusing them sends people down the wrong path:
- Conventional furnaces (59TP6, 58STA, Bryant 313AAV): the amber status LED behind the burner door blinks the code. Three blinks, pause, one blink is code 31. You count the flashes.
- Communicating Infinity/Evolution systems (59MN7): there is no LED to count. The fault is reported as a numeric code 31 in the wall control’s Last 10 System Events screen under the Service menu. Carrier’s own master fault list labels it “Furnace – High Pressure Switch Open.”
Either way the meaning is identical: the high-heat pressure switch did not close when the inducer started, or it opened during the cycle. The full code reference is on our Carrier furnace error codes page.
What actually causes code 31 in SoCal
From our service tickets across the LA basin, code 31 traces back to four things, in rough order:
- Clogged condensate trap on 90%+ condensing furnaces (the Infinity 59MN7 and most modern Performance units). The burner makes water; the trap drains it. Algae or debris blocks the trap, water backs up into the inducer housing or the pressure tap, and the switch cannot sense draft. The single most common cause we find.
- Blocked, sagging, or disconnected vent. A PVC intake/exhaust that sagged and pooled condensate, a bird screen packed with debris, or a vent termination too close to a wall. The inducer cannot move air against the restriction, so the switch never closes.
- Weak draft inducer. After eight idle months the inducer bearings stiffen and the wheel slows. It spins, but it no longer pulls enough negative pressure to close the switch. Common on 59TP6 and 59MN7 furnaces in year 8–12.
- The pressure switch itself out of spec, or a cracked/disconnected pressure tube to it. Real, but the minority of calls — which is why we meter before replacing.
Why it happens when it happens
An LA-basin furnace runs 200–400 hours a year against 1,500+ in a cold climate, and that long idle stretch from April to October is where code 31 breeds. The trap dries and cracks or grows algae, the vent collects debris, the inducer sits still and stiffens. The first November cold snap asks an untouched furnace to prove draft on its first fire in eight months, and the idle-season problems all surface at once. Mountain installs in Big Bear and Wrightwood run real heating hours and fail on a different curve — more inducer wear, fewer trap-and-idle faults. Coastal homes in Santa Monica and Malibu add salt-air corrosion to the vent and switch contacts. Fall maintenance in October is the cheapest insurance against a code 31 no-heat call; the related ignition-and-venting chain is walked through in our furnace short-cycling guide.
How we diagnose it — and what it costs
We do not jumper the switch to force the furnace to run, ever — that defeats a carbon-monoxide safety. We read the code, then work the draft chain: clear and test the condensate trap, check vent pitch and termination, meter actual draft pressure at the inducer, and meter the switch against its rated set point. Only then do we know which part is at fault. Pricing follows from the cause, and we quote SOT flat rates from our furnace tickets:
- Diagnostic: $89 ($149 after-hours), credited to the repair.
- Condensate trap clearing / vent correction: resolved at the diagnostic visit in most cases.
- Pressure switch: an inexpensive part — we replace it only when the meter proves it is out of spec.
- Draft inducer motor (59TP / 59MN): $580–$1,100, warranty-covered on the part if the system was registered within 90 days.
- IFC control board: $480–$950.
A registered Carrier furnace still carries labor on warranty parts ($200–$450 typical); we look up registration before ordering. For the full Carrier furnace pricing table, see our Carrier furnace repair page.
Repair or replace?
Code 31 by itself is not a replace-the-furnace verdict — a cleared trap or a corrected vent on an otherwise healthy furnace is a same-visit fix at any age. The judgment call comes when the cause is a failed inducer on an older furnace. A $580–$1,100 inducer on a 59TP6 at year 9 is worth fixing; the same repair on a 17-year-old 58STA that is also showing rust on the heat exchanger is throwing good money after bad. We give you the repair figure and a written replacement quote side by side. See furnace installation when replacement is the call, and our honest framing on furnace repair vs. replace.
Carrier and Bryant
Bryant and Carrier are the same corporation and largely the same equipment. The Bryant Legacy 313AAV is the Carrier 58STA under different badging; the IFC board, the inducer, and the pressure-switch logic cross-reference to the same parts and the same diagnosis. We service Bryant furnaces with the same meters and warranty channels as Carrier. For the cooling side of a Carrier system, see Carrier AC repair, and the full lineup on our Carrier brand page.