The short answer
A York furnace pressure switch trips when the inducer fan can’t prove the vent is clear, and the furnace won’t light until it can. In Northern Virginia the switch itself is usually fine. The common causes are a vent pipe blocked by snow, leaves or a nest, a condensate drain that backed up, a cracked hose on the switch, or an inducer motor wearing out. The light in the door window tells you how it failed: 3 red flashes means open when it should be closed, 2 means closed when it should be open, and 6 means it kept opening in the middle of a cycle.
What the switch is there for
Every time a York furnace starts, the inducer fan runs first and pulls air through the burners and out the vent. That suction pulls on a small diaphragm inside the pressure switch, which closes it and tells the board it’s safe to open the gas. No suction, no gas. It’s there so the furnace can never fire into a blocked vent and push exhaust into the house.
So when the switch trips, it’s almost always doing its job. Putting in a new one without finding out why the suction was low just moves the no-heat call a week down the road.
The $67 Heating Check
We check your furnace, heat pump or boiler before winter. Through December 31. See the offer
Which code you have
From York’s TM8E, TM9V and YP9C installation manuals.
Swipe to see all columns →
Swipe to see all columns →
| Light | What the board saw | Where to look |
|---|---|---|
| 3 red | Switch open when it should be closed | Vent, drain, hose and inducer, then the switch |
| 2 red | Switch already closed before the inducer started | A stuck switch or a wiring fault |
| 6 red | Switch opened four times in one call; locks out for an hour | A vent or inducer that’s barely making it |
| 13 red (TM9V) | High fire switch open when it should be closed | A partly blocked vent or a loose wire |
| 3 red (YP9C) | Switch open while the inducer is pulling hard | The switch and its pressure sensor |
| 5 amber (YP9C) | Still heating, but turned down for a restriction | A blocked vent or air pipe, or a clogged drain |
The causes, most likely first
A blocked vent or air intake
What you notice: 3 red flashes after snow, a windy fall day, or the first cold night after months off.
What causes it: High-efficiency York furnaces breathe through two white PVC pipes on an outside wall or the roof. Drifted snow, leaves, a bird or wasp nest, or an iced-over screen all cut the suction.
What you can do: Look at both pipes where they leave the house and clear anything in or around them by hand. Don’t push anything down the pipe.
When to call us: If they’re clear and it still trips, or if the pipes come out on the roof.
A backed up condensate drain
What you notice: 3 red flashes, sometimes with water under the furnace or a gurgle when it starts.
What causes it: A high-efficiency furnace makes water every time it runs. It drains through a trap and a hose, often to a small pump. When the trap clogs or the line freezes, water backs up where the switch reads and the switch can’t close.
What you can do: Check the clear trap and hose beside the furnace for sludge, and the condensate pump’s tank: full and humming means the pump quit. Our clogged or frozen drain guide goes further.
When to call us: If the trap is clean and the pump runs but the code comes back, or the pump is dead.
A cracked or loose hose
What you notice: 3 red flashes that come and go, or started after other work on the furnace.
What causes it: The switch reads suction through a rubber hose about the width of a drinking straw. Age cracks it, water can sit in a sagging one, and a hose that slipped off its port reads as no suction at all.
What you can do: Nothing to do safely with the power on. Mention it when you call.
When to call us: When the vent and the drain check out. A bad hose is a quick find with the door off.
A tired inducer motor
What you notice: The inducer sounds rough, grinding or louder than it used to, and the code shows up more on cold starts. Sometimes 6 red flashes instead of 3.
What causes it: Worn bearings slow the motor, and a slower motor pulls less air. York puts a faulty inducer first on its list for 3 and 6 flashes.
What you can do: Nothing; a slowing inducer needs a tech.
When to call us: Soon. An inducer that grinds is close to quitting completely.
The switch itself
What you notice: The vent is clear, the drain flows, the inducer sounds right, and it still trips.
What causes it: Switches do fail. It’s just usually the last thing on the list.
What you can do: Leave it alone. A bypassed switch lets the furnace fire into a blocked vent.
When to call us: When everything else checks out. A tech measures the suction the inducer makes against the rating printed on the switch and replaces it only if the suction is right and the switch still won’t close.
What you can do safely
- Clear the two PVC pipes outside.
- Check the trap, the drain hose and the condensate pump.
- Power cycle once. Set the thermostat to off. Turn off the breaker or the switch for the equipment, wait about five minutes, turn it back on and set the thermostat back. Do it once.
That’s the list. Don’t jumper the switch to get heat, don’t pull the hoses off to blow through them, and don’t keep power cycling. Three tries that end in 3 red flashes again tell you what one try did.
What a tech does on the visit
We read the last five faults off the board, then watch a start-up. We measure the suction at the switch with a manometer and compare it to the rating printed on the switch, check the hoses and the ports they connect to, flush the trap and the drain, look at the vent run inside and out, and listen to the inducer. You get the cause and the price before any part goes in.
Same switch, other brands
Every gas furnace has one: American Standard, Goodman, Lennox and Trane 3 flashes. The rest of the York chart is in our York furnace code guide, and every brand’s codes are in our error codes and warning lights guide. Got 7 or 8 flashes instead? That’s the flame sensor side. Our York repair page has what we fix and what it costs.
The system we put in first
Built to take a beating. Put in by our crew.
Heat lamps, snow and rain on one outdoor unit: that’s Trane’s own film. We’re a Trane Comfort Specialist, on Trane’s own dealer locator, and when you pick a Trane, our crew puts it in.
Films: Trane’s own.
Book a York repair
Call (703) 997-9757 or book online. A diagnostic visit is $99, and the tech reads the code, finds the part that set it and gives you the price before any work starts. We answer day or night. Our heating check is $67 per system, through December 31: furnace, heat pump or boiler. If someone already quoted you a new system, our free second opinion looks at it first. Rated 5.0 stars across 1,138 reviews on Google, Angi, Yelp, the BBB and Facebook, Virginia Class A license 2705192276.
Questions people ask
Can I bypass the pressure switch on my York furnace?
No. The switch is what stops the furnace from firing into a blocked vent. Bypassing it can push exhaust, carbon monoxide included, into the house. Find out why it’s tripping instead.
Why does my York pressure switch only trip on cold mornings?
Usually a drain line that freezes, a vent screen that frosts over, or an inducer that’s slow when it’s cold. If it’s a pattern, a tech can find which one before it turns into a no-heat night.
What does 6 red flashes mean on a York furnace?
The pressure switch opened four times while it was heating, so the furnace locked out for an hour. Same family of causes as 3 flashes, usually a vent or an inducer that’s barely making it.
Is a York furnace pressure switch expensive to replace?
The switch is one of the smaller parts in a furnace, and often it isn’t the problem. The $99 diagnostic finds out, and you get the repair price before any work starts.
