Quick answer
If your carbon monoxide alarm goes off, get everyone outside into fresh air right away and call 911. Don't go back inside until emergency responders say it's safe. Carbon monoxide has no color, taste, or smell, so treat every alarm as real until it's checked.
Key facts
| First step | Get everyone outside into fresh air |
|---|---|
| Then | Call 911 from outside |
| Why every alarm counts | CO has no color, taste, or smell (EPA) |
| Warning signs | Headache, dizziness, nausea, confusion, flu-like symptoms (EPA) |
| Common home sources | Furnaces, flues, gas water heaters, fireplaces, stoves, generators, car exhaust (EPA) |
What should I do when my carbon monoxide alarm goes off?
Get everyone out first, then call 911 from outside.
- 1Get everyone outside into fresh air right away, including pets.
- 2Call 911 from outside.
- 3Tell the dispatcher if anyone has a headache, dizziness, nausea, confusion, or feels sick.
- 4Stay outside. Don't go back in until emergency responders say it's safe.
- 5Don't run your furnace or other gas equipment again until the source is found and fixed.
Don't silence the alarm and stay inside. Even if no one feels sick, get out and call.
What is carbon monoxide, and where does it come from?
Carbon monoxide, or CO, is a toxic gas with no color, taste, or smell, according to the EPA. Because you can't sense it, an alarm may be your only warning.
The EPA lists these among the sources of CO in homes:
- Leaking chimneys and furnaces
- Back-drafting from furnaces, gas water heaters, wood stoves, and fireplaces
- Gas stoves, and unvented gas or kerosene space heaters
- Generators and other gasoline-powered equipment
- Car exhaust from an attached garage
- A furnace flue that's blocked, disconnected, leaking, or the wrong size
What are the signs of carbon monoxide poisoning?
The EPA lists headaches, dizziness, confusion, nausea, fatigue, and flu-like symptoms that clear up after leaving home. At very high levels, CO can kill.
Effects vary a lot from person to person, depending on age, health, and how much CO is in the air and for how long. If anyone feels sick, tell 911.
What happens after a carbon monoxide alarm?
Once responders say it's safe, the source needs to be found and fixed before gas equipment runs again. If a furnace, flue, or other gas-burning equipment is the cause, it needs a repair and a safety check.
If your furnace may be involved, a heating technician can inspect the burners, heat exchanger, flue, and venting. ENERGY STAR notes that a dirty burner or cracked heat exchanger can make heating equipment run less safely.
How can I lower the risk of carbon monoxide at home?
A few habits cover the most common sources.
- Have a trained professional inspect, clean, and tune up your furnace, flues, and chimney every year, as the EPA recommends.
- Put CO alarms on each level of your home, outside each sleeping area. The EPA points to this recommendation from the Consumer Product Safety Commission.
- Never run a generator inside your home or garage, even with doors and windows open.
- Don't idle a car in the garage.
- Test your CO alarms and change batteries as the maker directs.
Common questions
Can a furnace cause carbon monoxide?
Yes, a furnace can cause carbon monoxide. The EPA lists leaking furnaces, back-drafting, and blocked or disconnected flues among the sources of CO in homes. That's why the EPA recommends a yearly professional inspection of furnaces, flues, and chimneys.
What if my CO alarm goes off and then stops?
Treat an alarm that stops as real: get everyone outside and call 911 anyway. An alarm that stops doesn't mean the CO is gone.
Where should carbon monoxide alarms go?
Put one on each level of your home, outside each sleeping area. That's the Consumer Product Safety Commission's recommendation, which the EPA cites. Follow the alarm maker's instructions for placement and testing.
Can I open windows and stay inside?
No, get everyone outside and call 911 instead. Opening windows doesn't find or fix the source, and you can't tell from inside how much CO is in the air.
When can I use my furnace again after a CO alarm?
Only after responders say it's safe and the source is found and fixed. If the furnace was the cause, or might be, have a technician inspect and repair it first. Don't restart it to test.
Sources
- Carbon Monoxide's Impact on Indoor Air Quality — U.S. EPA. Checked 2026-09-24.
- Maintenance Checklist — ENERGY STAR (U.S. EPA). Checked 2026-09-24.
Last updated: · Reviewed by: HVAC Repair Guys