Furnace Blowing Cold Air: Thermostat & Flame Sensor Fixes

Quick Answer

A furnace blowing cold air is often caused by a thermostat set to ON instead of AUTO, causing the blower to run between cycles. The primary fix is switching the thermostat to AUTO or cleaning a dirty flame sensor that prevents ignition.

Cold air from your furnace usually means one of two things: the blower's running when it shouldn't be, or the burners light and shut right back off because the flame sensor can't confirm combustion. If you ignore a dirty flame sensor long enough, the control board locks out completely and you've got zero heat. I see this knock out heating on the coldest nights of the year way more than you'd think.

GenericFurnaceSeverity: moderateDifficulty: intermediate75% DIY Success

Time to Fix

15–90 min

Difficulty

intermediate

Parts Cost

—

Tools Needed

Phillips #2 screwdriver, 1/4 inch nut driver or hex driver set

What Does the BLOWING-COLD Code Mean?

OK so most of the time when I show up for this call, it's one of three things: thermostat's set wrong, the flame sensor is coated in oxidation and can't confirm a flame, or the filter hasn't been changed in six months and the limit switch tripped to protect the heat exchanger. The fix is usually under $30 in parts if you DIY it. But if you've been ignoring it long enough, you might've stressed the heat exchanger, and that's a whole different conversation.

Most Likely Causes

Based on aggregated repair data, here is the probability breakdown for this error code:

Component failure in the Generic furnace40%

Sensor or thermostat out of operating range24%

Control board fault14%

Power or electrical supply issue12%

Mechanical wear requiring inspection10%

Symptoms You May Notice

Air coming out of every vent is room temperature or cold even though the thermostat is set to heat and the display shows it's calling for heat.
Furnace kicks on, you feel a brief burst of warm air for maybe 30 seconds, then it goes cold and the furnace shuts down before the house ever reaches the set temperature.
The blower runs constantly without stopping, even hours after the thermostat should've been satisfied, blowing lukewarm air non-stop.
You can hear the furnace try to ignite two or three times in a row, clicking and attempting to light, then it gives up and goes quiet while the fan keeps running.
House takes two or three times longer than usual to heat up and never actually reaches the set temperature on a cold day, even with the furnace running almost continuously.

Can you reset a Generic furnace to clear the BLOWING-COLD code?

Find the service disconnect switch on the side of the furnace cabinet. It looks exactly like a wall light switch. Flip it off. Wait a full 60 seconds, not 10, not 30, a full minute so the control board completely clears its lockout memory. Flip it back on. The furnace should attempt a fresh ignition sequence within 30-60 seconds. Watch the startup so you can see exactly where it fails if it fails again.

Tools Required for Diagnosis

Phillips #2 screwdriver1/4 inch nut driver or hex driver setMultimeter (set to microamps if testing flame sensor signal)Fine emery cloth or a dollar bill (for cleaning flame sensor rod)FlashlightReplacement air filter (size is printed on your current filter's frame)

Diagnostic Checklist

Follow these steps in order. We start with the easiest external fixes before opening up the machine.

Frequently Asked Questions

How much does it cost to fix a furnace blowing cold air?

If it's the thermostat setting, free. Cleaning the flame sensor yourself costs nothing but 20 minutes of your time. A replacement flame sensor is usually $15-30 online. A new air filter is $5-20. If you're calling a tech, expect $150-300 for the service call and basic repair. They'll add $50-100 if they need to swap the sensor or reset and test the limit switch. The expensive scenario is if you ignored it so long the heat exchanger cracked from repeated overheating. That's $800-1500 depending on the unit, and at that point you're probably looking at replacement.

Is it safe to diagnose this myself?

Honestly, yes for most of it. Checking the thermostat is completely safe. Cleaning the flame sensor is totally fine as long as you kill both the gas and the power first. Where I'd stop and call someone is if you smell gas near the furnace while it's trying to ignite, or if the igniter doesn't glow at all after a reset. No glow could mean a bad igniter, a bad control board, or a wiring issue. And if you smell gas at any point, close the gas valve, open a window, get out, and call the gas company before you call an HVAC tech.

Why does my furnace blow cold air only sometimes and not every time?

Intermittent cold air is almost always a flame sensor that's borderline dirty. On a cold morning when the furnace has been off all night, the oxidation on the sensor is slightly worse and the microamp signal doesn't get through. After a couple of restart attempts or once the furnace warms up, it works fine. It'll get more frequent until it fails completely. Clean the sensor now and you'll probably get another season out of it. If cleaning it doesn't stop the intermittent trips, the sensor rod itself might be cracked or the wire connector corroded.

How often should I be changing the furnace filter?

Basic 1-inch fiberglass filters need to come out every 30 days during heating season. Thicker 4-inch media filters, every 6-12 months. Here's something I see constantly: people buy MERV 13 filters because they figure better filtration is always better. But a higher MERV rating restricts airflow more, and if you leave that filter in for six months, you're basically choking the furnace. The high-limit switch trips and you're standing in a cold house wondering what happened. Match the filter to what the furnace manufacturer specifies, not what the filter packaging says is best.

What if the burners are staying lit but the house still won't get warm?

If you've confirmed the burners stay lit and the limit switch isn't tripping, the heat is probably escaping before it reaches the rooms. Go into the basement or crawlspace while the heat's running and run your hand along the main duct trunk lines. Feel for warm air leaking out at seams or where flex duct connects to the plenum box. Even a 2-inch gap at a joint can dump a huge amount of heated air into an unconditioned space. Also check every return air grille in the house. A couch pushed against a return vent will absolutely starve the system of the airflow it needs to heat properly.

Models Known to Experience BLOWING-COLD Errors

This repair applies to most Generic furnaces with this error code. Common model numbers include:

Carrier 59SC5A080S17, Lennox SL280UH090V36B, Trane XR95 TUD2B080A9V3VA, Goodman GMVC96 GMVC960804CN, Rheem RGFG-07EAMER, American Standard Silver 95 2TUC2B080A3HVA, Bryant 926TB48080V17

Written by

Raj Patel

HVAC & Water Systems Specialist · 15 years experience

Last verified for technical accuracy on March 15, 2026