The short answer
The pressure switch on a Carrier furnace in Northern Virginia is a small round switch with a rubber tube on it, near the inducer motor on top. It only closes once the inducer has pulled enough draft through the vent pipe, and the furnace won’t light until it does. When it won’t close you get code 31 or 32, and when it won’t open you get code 23. The switch itself is rarely what failed. It’s reporting a plugged drain, a blocked vent, a bad tube or a tired inducer.
What you’ll notice
- The inducer runs, then the furnace stops before the burners light.
- The light behind the lower door flashes 31, 32 or 23.
- A gurgle from the furnace, or water around it.
- It works fine in mild weather and fails on the coldest nights.
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How the switch works
When the thermostat calls for heat, the inducer starts and pulls exhaust out through the vent. That pull makes a slight suction in the tube to the pressure switch, and the switch closes. Only then does the board turn on the igniter and the gas. The switch is a proof that the exhaust has somewhere to go. Two-stage Carrier furnaces have two switches, one for low heat and one for high.
The codes it throws
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| Code | What Carrier’s manuals say | Where to look |
|---|---|---|
| 31 | The pressure switch didn’t close, or it reopened during the cycle (high-heat switch on a two-stage) | Drain, vent, tube, inducer |
| 32 | The low-heat pressure switch didn’t close, or it reopened | Same as 31 |
| 23 | The pressure switch didn’t open when the inducer stopped | A stuck switch or a blocked tube |
On a 31, if the switch stays open more than 5 minutes, Carrier’s board shuts the inducer off for 15 minutes and then retries. Our posts on code 31 and codes 21, 22 and 23 go code by code.
Why a new switch rarely fixes it
A pressure switch is a cheap part, so it’s the first thing people buy. But if water is backing up from a plugged condensate trap, or the vent is blocked, or the inducer is too weak to pull the draft, a new switch sees exactly what the old one saw and trips the same way. The causes Carrier lists for a 31 are the condensate drain and trap, the vent pipe, the tube, the inducer, wind at the vent ends, the gas pressure and the combustion air. The switch is near the end of that list.
What you can safely check
- Look at the white PVC pipe ends outside. Clear leaves, snow or ice from around them, and don’t push anything into the pipe.
- Look at the condensate hose and pump beside the furnace. A full pump that isn’t running is a strong lead, and our condensate drain guide walks through it.
- Find the rubber tube on the switch. If it slipped off its port, push it back on. If it’s cracked, kinked or has water in it, leave it for us.
- Count the code, then turn the furnace off for 30 seconds and back on to let it try once.
What not to do
Never jump or bypass a safety switch to get heat back. Each one is there to stop the furnace from burning gas when something is wrong, and the wrong thing is often exhaust, which carries carbon monoxide. Jumpering a pressure switch is how exhaust ends up in the house. If your carbon monoxide alarm goes off, get everyone outside and read our CO alarm guide from there.
What a technician does
Carrier’s manuals have a component test: disconnect the inducer, call for heat, and wait a minute. The igniter must not glow, and the board should flash 31. That proves the switch opens when it should. Then we put a manometer on the tube and compare the draft the inducer actually pulls with the setting printed on the switch. If the draft is there and the switch won’t close, it’s the switch. If the draft is low, we find out why: the trap, the vent, the inducer. When we put a tube back, we make sure it has no sags or kinks where water can sit.
What it costs to have us look
A repair visit starts with a flat $99 diagnostic, and you get the price of the fix before we touch anything. Our Carrier repair page lists what the common repairs cost. Our heating check is $67 per system, through December 31: furnace, heat pump or boiler.
Related: every Carrier furnace code, how to reset a Carrier furnace, the Goodman version of this problem and our error code and warning light guide.
Every code meaning here comes from Carrier’s own documents: 58SC0A installation instructions, 58TP0B manual and 59MN7C manual.
Call or book
Call (703) 997-9757 and tell us the code you counted, or book online from goswiftpro.com/cookiesoffer and a box of warm cookies comes with the visit. Rated 5.0 stars across 1,138 reviews on Google, Angi, Yelp, the BBB and Facebook, Virginia Class A license 2705192276.
Questions people ask
What does the pressure switch do on a Carrier furnace?
It proves the inducer is pulling exhaust out through the vent before the furnace lights. No draft, no ignition.
What are the signs of a bad pressure switch?
Code 31, 32 or 23 on the light, and an inducer that runs while the burners never light. Most of the time the switch is fine and the drain, vent, tube or inducer is the real problem.
Can I replace a Carrier pressure switch myself?
We don’t recommend it. A new switch won’t fix a plugged drain or a weak inducer, and the right switch depends on the model. We measure the draft first.
Can a clogged condensate drain trip the pressure switch?
Yes. Water backing up into the furnace is one of the most common reasons it won’t close, especially on cold nights.
How much does a pressure switch repair cost?
The visit starts with a flat $99 diagnostic, and you get the price of the repair before any work starts. Our Carrier repair page lists the common repairs.
