Carbon monoxide alarm going off or beeping

A carbon monoxide (CO) alarm that is sounding is treated as real until responders say otherwise, so the first question is about the alarm and how everyone feels. Short chirps are a different signal, and the pattern depends on the maker. Let's sort out which one you have.

Also searched as: carbon monoxide detector going off, carbon monoxide alarm beeping, carbon monoxide detector going off but no gas appliances.

Before you start. If a CO alarm is sounding, or anyone feels dizzy, sick or has a headache, get everyone outside to fresh air now, pets included, and call 911 from outside. Do not go back in until responders say it is safe.

Every question and answer on this page, in order

Question 1: What is happening right now?

Pick the first one that fits. If you are not sure, pick the first one.

The alarm is sounding loudly, it sounded and then stopped, or its display shows a CO number: Emergency: get outside and call 911
Someone, or a pet, has a headache, dizziness, nausea, weakness or confusion, with or without an alarm: Emergency: possible CO poisoning
The alarm only chirps, and nobody, pets included, feels unwell: go to question 2
Responders already checked the home and said it is safe: Responders cleared it: have the source checked

Question 2: How does the alarm sound?

If you hear more than one beep in a row, or you are not sure, choose one of the last two answers right away. Otherwise, listen for a full minute and read the label for the brand. Chirp patterns differ by maker, so your manual has the final word.

One chirp about every minute (Kidde says this may be a low battery, and First Alert lists it as low battery): go to question 3
One chirp about every 30 seconds (Kidde lists this as end of life or a fault): Likely cause: end of life or a malfunction
Two or more chirps in a row, then a pause, repeating: Emergency: get outside and call 911
Four beeps, loud or soft, a pause, then four more, or I cannot tell what the sound means: Emergency: get outside and call 911

Question 3: Have you already put in a fresh battery of the type your manual lists?

Only change the battery the way your alarm's manual shows. If your alarm is hardwired, leave the battery alone and choose Not yet, and the next page explains who to call.

Not yet: Likely cause: a low battery
Yes, and it still chirps: Emergency: get outside and call 911

Possible results

Need the words for the phone call? Build a what-to-tell-the-technician checklist, or read when to call an hvac pro.