Furnace Setting Off the CO Alarm in Winter? Repair Costs

First things first: get out, then read this

If your CO alarm is sounding right now, stop reading and get everyone outside. Call 911 or your gas utility from the yard. Come back to this page once the air is cleared.

Now the money question.

If the furnace is the actual source, plan on $150–$700 for professional diagnosis plus common repairs like clearing a flue or cleaning burners.

A cracked heat exchanger is the expensive outlier. That repair can run $1,200 to $2,500, and on an older furnace it often tips you toward full replacement.

The good news: a lot of winter CO alarms trace back to a $40 detector that hit its expiration date. The bad news: you can't assume that. You have to rule out the deadly stuff first.

What you'll actually pay in 2026

Let's break the numbers down by scenario so you know what's fair.

A decent plug-in or battery detector runs $25 to $50 each, and you should have more than one in the house anyway.

Pro diagnosis, $120 to $220: a service call with a combustion analyzer to measure CO at the registers and inspect the heat exchanger. Some companies waive the fee if you do the repair with them.

Common pro repairs, $150–$700 all in: reseating or clearing a flue, cleaning and adjusting burners, or replacing a bad inducer motor or pressure switch that caused poor venting.

Heat exchanger replacement, $1,200 to $2,500: the part is buried deep in the furnace, so labor is heavy. On a unit over 15 years old, most techs will steer you toward replacement instead.

Full furnace replacement, $4,500 to $8,000: worth considering when the heat exchanger is cracked, the furnace is old, and the part is expensive or discontinued.

• New CO detector: $25 to $50 each
• Fresh batteries: $8 to $15
• Pro diagnostic with combustion analyzer: $120 to $220
• Flue clearing / burner cleaning: $150–$700
• Heat exchanger replacement: $1,200 to $2,500
• Full furnace replacement: $4,500 to $8,000

How we put these numbers together

Our cost ranges come from real 2026 service pricing across US HVAC contractors, plus retail pricing on CO detectors and common furnace parts.

The DIY figures assume you're buying quality detectors and batteries, not the cheapest thing on the shelf.

Pro ranges reflect a standard service call with a combustion analyzer plus the labor for common venting and burner repairs. Heat exchanger and full-replacement numbers reflect national averages and swing with your region and furnace size.

We lean conservative on safety. When a repair touches CO, we'd rather point you to a licensed tech than save you a service fee.

Prices vary by market, furnace brand, and how buried the failed part is. Get a written estimate before you approve any furnace repair over a few hundred dollars.

What's actually setting off the alarm

Here's how I sort it in the field, roughly in order of how often you'll see each one.

Expired detectors top the list. CO sensors have a hard lifespan, usually 7 years, and they get twitchy near the end. Flip the unit over and read the manufacture date.

After that comes the scary category: real CO from the furnace. A cracked heat exchanger, a disconnected or blocked flue, or dirty burners burning rich can all push carbon monoxide into your air.

Snow-blocked vents are a pure winter problem. High-efficiency furnaces vent through PVC out a sidewall, and a snowdrift can plug that pipe fast.

Don't forget sources that have nothing to do with the furnace. A car warming up in an attached garage or a backdrafting water heater can trip the same alarm.

• Detector past 7 years old (most common)
• Cracked heat exchanger (most dangerous)
• Blocked or disconnected flue/vent
• Dirty burners causing bad combustion
• CO from a garage, generator, or another appliance

The order to work through this

Safety comes before diagnosis. Every time.

Once the fire department or utility clears the air and gives you the all-clear, turn the furnace off at its dedicated switch (looks like a light switch, usually near the unit) and set the thermostat to off.

Then check the cheap stuff. Read the detector's date. If it's expired, replace it with a fresh unit and test. If a brand-new detector alarms too, you've got a real source and you're calling a pro.

While you wait, look outside for a buried vent. Clear snow away from the exhaust pipe if you can reach it without climbing anything sketchy.

Keep the heat off until a tech confirms it's safe. Use electric space heaters (plugged directly into the wall, not a power strip) for warmth.

What you can do yourself and where to stop

The DIY line here is bright and easy to see. You handle the detector. A pro handles the furnace.

Replacing a CO detector, swapping batteries, and clearing snow off an outdoor vent are all fair game. So is ruling out a car or generator running near the house.

Everything past that needs a licensed HVAC tech with a combustion analyzer. You cannot eyeball a cracked heat exchanger, and a hairline crack still leaks CO.

I'll say it plainly: don't try to inspect the heat exchanger yourself, don't reseal flue joints and call it good, and don't adjust burners. This is the category of repair where a mistake ends in the ER.

If the alarm keeps going after a fresh detector, the furnace stays off and the pro gets called. No middle ground.

Can this wait until morning?

If the alarm is actively sounding, no. Not one minute. CO poisoning can kill before you feel more than a headache.

If you've swapped in a fresh detector and it stays quiet, and the furnace is off, then yes, you can wait for a normal-hours appointment. The risk is gone once the furnace isn't running.

The danger is the in-between move people make: silencing the alarm and running the furnace anyway to stay warm. That's the mistake that fills obituaries every winter.

Cold house overnight is uncomfortable. It's also survivable with blankets and an electric space heater. A CO leak is not something you sleep through and fix later.

When in doubt, leave the furnace off and call in the morning. A cold night beats the alternative.

Carbon monoxide safety, no shortcuts

CO is colorless, odorless, and it doesn't warn you. Your detector is the only thing standing between you and a gas you literally cannot sense.

Learn the symptoms: headache, dizziness, nausea, confusion, weakness. If more than one person or a pet feels sick at once and gets better outside, that's CO until proven otherwise.

Put detectors on every level and near every bedroom. Keep them at least 15 feet from the furnace so normal startup exhaust doesn't cause nuisance alarms.

Replace every CO detector at 7 years, full stop. Write the replace-by date on it with a marker the day you install it.

Never heat with the oven, a grill, or an unvented gas heater indoors. Every winter those shortcuts send people to the hospital.

How your furnace's age changes the math

The furnace's age drives the repair-versus-replace decision more than anything else.

Furnaces under 12 years old are usually worth fixing. If it's a flue or burner issue, you repair it and move on. Even a heat exchanger repair can pencil out under warranty.

Once you're past 15 years and staring at a cracked heat exchanger, the numbers shift. You're spending $2,000-plus on a unit near the end of its life, and the rest of it will start failing soon after.

Homes built before the mid-2000s often have 80 percent efficiency furnaces that vent through metal flue pipe into a chimney. Those flues can corrode and separate over decades.

Newer high-efficiency units vent through PVC out a sidewall, which is where snow blockage becomes the winter culprit. Different system, different failure, same alarm.