THE FIRST ACTION
If a carbon monoxide alarm sounds, get everyone outside to fresh air immediately and call 911 or the fire department from outside. Do not wait for symptoms and do not go back inside until responders say it is safe.
Stop. Put safety first.
Leave first. Do not stay inside to find the source, silence the alarm, look up a model, open windows, or shut off appliances. If someone is still inside, tell responders; do not re-enter to attempt a rescue.
Before you begin
Immediate action for a sounding CO alarm or suspected exposure. This page does not interpret universal beep patterns, diagnose poisoning, or authorize re-entry.
Nothing that delays leaving. Once safely outside, tell responders the address, the alarm or concern, and whether anyone has symptoms or remains inside.
In a rental, report the problem to your landlord or property manager before making repairs. Building rules and local requirements vary.
What to do next
- 01
Move to fresh air
Leave the building promptly using a safe exit and move away from the hazard. Carbon monoxide cannot be reliably detected by smell or sight. Do not interpret the absence of an odor as reassurance.
- 02
Call for emergency help from outside
Call 911 or the fire department and follow the dispatcher’s instructions. Tell them if anyone has a headache, dizziness, weakness, nausea, chest pain, or confusion. Do not wait for all of these signs or try to diagnose the cause yourself.
- 03
Stay out until responders clear the building
A quiet alarm after leaving does not establish safety. Let responders check the environment and decide about re-entry. Tell them if fuel-burning equipment or a generator was operating, if you already know.
- 04
Arrange follow-up only after the emergency is resolved
Follow the responder’s instructions for equipment inspection before reuse. From a safe setting, consult the exact alarm manual about maintenance, age, replacement, and any separately identified low-battery signal. Do not remove batteries as a response to a suspected CO alarm.
What your result means
These observations help choose a next step. They do not confirm a diagnosis.
| What you notice | What it tells you | Next step |
|---|---|---|
| A CO alarm is sounding or exposure is suspected | Treat it as an emergency. | Get outside and call emergency services. |
| You are unsure whether the sound means CO | The signal has not been safely identified. | Take the safe course: leave and ask emergency services for guidance. |
| Responders have cleared the incident | Follow-up may still be needed. | Use their instructions and qualified service before restarting implicated equipment. |
Know what happens next
Before you resume normal use
Only the responding authorities can advise on safe re-entry after the incident. An online checklist, lack of symptoms, open windows, or an alarm that stopped is not a clearance.
When to call a professional
Do not investigate a combustion appliance, vent, flue, generator, or gas connection yourself. Medical symptoms after suspected exposure require urgent medical assessment; follow emergency advice.
Take useful notes with you
A short record makes it easier to explain the problem to a landlord, technician, or responder. Do not collect notes during an emergency.
Notes stay in this page until you leave or reload. Download a copy to keep them. They are not sent to us.
Common questions
Nobody feels sick. Should we still leave?
Yes. Do not wait for symptoms when a CO alarm sounds.
Can I check the batteries first?
Do not delay leaving a suspected CO incident to check the device. Maintenance and signal identification come later, from a safe setting.
Sources & scope
This is a desk-researched guide, not a hands-on test or an on-site diagnosis. A licensed tradesperson has not independently reviewed this edition. Follow your product’s manual and local requirements.
- CPSC: Get to Know Carbon Monoxide Alarms
Supports getting outside immediately and calling the fire department.
- CDC: carbon monoxide poisoning basics
Supports CO’s lack of color/odor, possible symptoms, and qualified servicing of fuel-burning equipment.
Published 2026-09-24 · Our editorial policy · Report a correction