Why is my furnace blowing cold air with a red blinking light?
A furnace blowing cold air and flashing a red error code often indicates a tripped pressure switch or an issue with the inducer motor preventing ignition. This stops the unit from heating your home, leaving you with unheated air running continuously. Fortunately, some underlying causes like blocked tubing can be checked and fixed at home.
What the numbers show
Counted from the threads quoted below. How we count
- People have been saying this in most months since September 2026.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
Found another way around this?
Share your steps and trade-offs so fellow engineers don't have to reinvent the wheel. No account required.
Why is my furnace blowing cold air with a red blinking light?
- 1Replacing the pressure switch entirely2 people mentionedDid this help you?
- 2Checking control board voltage with a multimeterDid this help you?
- 3Manually blowing into the pressure switch hose to test the switchDid this help you?
- 4Inspecting intake and exhaust vents for blockagesDid this help you?
- 5looking up the error code on the back panel or diagnosis chartDid this help you?
- 6having a reputable company do a safety check and maintenanceDid this help you?
- 7Checking hoses for blockagesDid this help you?
- 8Removing and draining the inducer housingDid this help you?
- 9Calling a professional technicianDid this help you?
- 10Replaced the filterDid this help you?
- 11Shutting the furnace off at the breaker for 5 minutes to reset safety switchesDid this help you?
- 12Disconnecting the pressure switch tubing and blowing lightly into it to clear it or test the clickDid this help you?
- 13Checking and draining water from the thicker tube connected to the motorDid this help you?
- 14Checking the outside exhaust pipe for debris or ice blockagesDid this help you?
- 15Jumpering across the switch to see if the error code changes.Did this help you?
- 16Replacing the inducer motor and switch without checking for voltage first.Did this help you?
- 17Disconnecting the positive hose to force the switch to closeDid this help you?
- 18Clearing the traps and portsDid this help you?
- 19Dropping an object down the exhaust to check for physical blockagesDid this help you?
In their words
Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.
“Hi! Ive got a Concord 90 plus high efficiency furnace from 2009 that's dx light blinks red 3 times. Is this something simple that could be fixed?”source ↗
“The furnace spins up higher then higher the. Stops. There are three red flashes coming from the panel. After some searching it says there’s a pressure switch failure.”source ↗
“This morning my wife tried turning on the furnace and it was “only blowing cold air”.”source ↗
“I’m receiving T045_F lo press.switch open, inducer on hi speed and T057_F hi press.switch open, inducer on high speed errors on a Rheem prestige gas furnace.”source ↗
“My furnace is only blowing air. It's flashing green twice and then red five times.”source ↗
“I'm a little nervous to mess with things because of gas”source ↗
“My apologies as I’m very inexperienced with this. There’s a black tube that’s connected to the top of the switch, and then the tube at 7 o’clock that looks like it connects back into the back of the switch. Which end do I disconnect and blow?”source ↗
“I've blown lightly into all the small tubes and after a couple tries it started working again. would replacing the pressure switch eventually be the next move?”source ↗
“Code was “Pressure switch stuck open”. So I checked the sensing line to that switch and it seemed intact.”source ↗
“The inducer has never turned on at any point when I’ve watched and listened to it during start up.”source ↗
“I’m never getting 120 during the sequence actually at the inducer. So I’m thinking it’s just never getting that switch to switch because there’s no draft being created by the inducer motor. Which is because the board is never sending voltage there.”source ↗