Furnace Blowing Cold Air? 9 Causes and How to Fix Them
A furnace blowing cold air usually has one of three problems. The thermostat fan is set to "On" instead of "Auto." A clogged filter has made the furnace overheat and shut its burners off. Or the burners won't light at all, because of a dirty flame sensor, a failed igniter or a blocked drain. Check the first two yourself in five minutes for free. If the burners still won't stay lit, most repairs cost $100 to $400.
Typical pro cost
$100 to $400
- Urgency
- Same day in freezing weather
- DIY difficulty
- Easy checks first
- Time to fix
- 5 minutes to 2 hours
- Prices
- US typical range
Last reviewed October 2026
Why is my furnace blowing cold air? Check these two things first
Before you call anyone, rule out the two causes that HVAC techs find on service calls every single week. Both take less than five minutes.
- The thermostat fan setting. If the fan is set to "On," the blower runs nonstop, including between heating cycles when the burners are off. You'll feel cool air blowing for long stretches. Switch it to "Auto," so the blower only runs while the furnace is actually heating.
- The air filter. A clogged filter chokes the airflow. The heat exchanger gets too hot, and a safety switch called the high-limit switch shuts the burners off. The blower keeps running to cool things down, and that's the cold air you feel. Replace the filter. Then turn the furnace off for 15 minutes so the limit switch can reset.
Also make sure the thermostat is on "Heat," and set a few degrees above the room temperature. A dead battery, or a schedule that dropped the setpoint overnight, catches a surprising number of people.
When the furnace blows cold air tells you why
- For the first minute after it turns on: Normal. The burners light first, and the blower waits 30 to 90 seconds for the heat exchanger to warm up. Some systems start the blower a little early.
- For a minute or two after the heat shuts off: Also normal. The blower keeps running to pull the last heat out of the furnace.
- Warm at first, then cold, then warm again: The furnace is overheating and tripping the high-limit switch. It's usually a clogged filter, closed vents, or a dirty blower.
- Cold the whole time, with clicking and no flame: An ignition problem. Possible causes are the igniter, the flame sensor, the gas supply, or a blocked vent or drain.
- Burners light for a few seconds, then go out: Almost always a dirty flame sensor. It's one of the cheapest furnace repairs.
- Warm air at the furnace, cold at some vents: Leaky or disconnected ductwork in an attic or crawl space.
9 reasons a furnace blows cold air
1. Fan set to "On"
The blower runs between heating cycles, so you feel room-temperature air that seems cold. Set it to "Auto."
2. Clogged air filter
Restricted airflow trips the high-limit switch. The burners shut off while the blower keeps running.
3. Dirty flame sensor
The burners light, then shut off within seconds because the sensor can't "see" the flame through its coating of residue.
4. Failed igniter
Hot-surface igniters crack with age. No glow means no flame, and after a few tries the furnace locks out.
5. Gas supply problem
A closed gas valve, an unpaid bill, or a utility outage. Check whether your stove or water heater works.
6. Pilot light out (older furnaces)
Furnaces with a standing pilot won't heat if it's out. A pilot that won't stay lit usually needs a new thermocouple.
7. Clogged condensate drain
On high-efficiency furnaces, a blocked trap or drain trips a safety switch and stops the burners. Look for water in or around the furnace.
8. Blocked vent or inducer problem
Snow, ice or a nest at the outdoor vent pipes, or a failing inducer motor, stops the pressure switch from letting the burners light.
9. Leaky ductwork
Heat escapes into the attic or crawl space before it reaches the room. The furnace works fine, but the far rooms stay cold.
How to fix a furnace blowing cold air, step by step
- 1
At the thermostat, set the mode to "Heat," the fan to "Auto," and the temperature 3 to 5 degrees above the current room temperature. Replace the batteries if the screen is dim.
- 2
Replace the air filter. If it was very dirty, turn the furnace off at its power switch for 15 minutes so the high-limit switch can reset. Then turn it back on.
- 3
Open every supply vent and make sure return grilles aren't blocked by furniture or rugs. Closing too many vents can make the furnace overheat.
- 4
Check that the gas valve on the pipe going into the furnace is open. The handle runs parallel to the pipe. Check that other gas appliances work.
- 5
Go outside and clear any snow, ice or debris from the furnace's intake and exhaust pipes. On high-efficiency furnaces these are usually two white plastic pipes.
- 6
Look at the status light through the small window on the furnace door. Count the blinks and match them to the code chart inside the door. Write the code down for the technician.
- 7
Watch one start-up. If the burners light and go out after a few seconds, the flame sensor likely needs cleaning. If there's no glow or spark at all, the igniter or control may have failed. Either way, it's time for a technician unless you're comfortable working inside the furnace.
Tools and materials
- New air filter (size printed on the old one)
- Fresh thermostat batteries
- Flashlight
- Phone to record the error code
Approximate DIY cost: $5 to $40
Call a pro when
- The burners won't light, or go out after a few seconds
- The error code points to ignition, pressure or limit switches
- The furnace keeps overheating even with a clean filter
- You smell gas or exhaust, or a CO alarm sounds
- Rooms stay cold even though the furnace runs normally
Cost to fix a furnace blowing cold air
| Job | Typical cost |
|---|---|
| New air filter (DIY) | $5–$40 |
| Diagnostic or service call | $100–$200 |
| Clean or replace flame sensor | $80–$250 |
| Replace hot-surface igniter | $100–$400 |
| Replace thermostat | $85–$300 |
| Replace high-limit switch | $150–$350 |
| Clear condensate trap and drain | $75–$250 |
| Furnace control board replacement | $300–$700 |
| Seal or reconnect leaky ductwork | $300–$1,500 |
Ranges are typical US prices compiled by FixHomeCost from published cost guides. Emergency and weekend rates in cold snaps are higher. A fall tune-up that cleans the flame sensor and checks the igniter prevents most of these calls.
Furnace or heat pump? Make sure you know which you have
If your outdoor unit runs in winter, you may have a heat pump, not just a furnace. Heat pumps blow air that feels cooler than a furnace's, and they briefly blow cool air during defrost. That's normal. See our guide to a heat pump blowing cold air if that's your setup.
Common questions
Why is my furnace blowing cold air?
The most common reasons are a thermostat fan set to "On," a clogged air filter that made the furnace overheat, or burners that won't light because of a dirty flame sensor, a failed igniter or a blocked drain or vent.
How do I reset my furnace when it's blowing cold air?
Replace the filter, then switch the furnace off at its power switch or breaker for 15 minutes, and turn it back on. That clears many lockouts and lets the high-limit switch reset. If it keeps blowing cold, call a technician.
Can a dirty filter make a furnace blow cold air?
Yes. A clogged filter makes the furnace overheat. The high-limit switch then turns the burners off while the blower keeps running, which sends cool air through the vents.
Why does my furnace blow warm air then cold air?
That cycle usually means the furnace is overheating and tripping its limit switch. Check the filter and vents first. If it continues, the blower may be dirty or the heat exchanger may need inspection.
How much does it cost to fix a furnace blowing cold air?
Most repairs cost $100 to $400, such as a flame sensor, igniter or limit switch. A control board runs $300 to $700. Fixing a filter or thermostat setting yourself costs little or nothing.
Is it normal for a furnace to blow cold air at first?
A few seconds of cool air at startup is normal on some systems, and the blower runs for a minute or two after the burners stop. Cold air for the whole cycle is not normal.
Related guides
This is general information, not a professional inspection. Gas furnace work can be dangerous; hire a licensed professional when in doubt. See how we research costs.