Carbon Monoxide Detector Going Off? Leave First, Then Read This

Like our gas-smell page, this one starts with no troubleshooting. Carbon monoxide (CO) has no smell and no color, and the only warning you get is the detector. If it's alarming, believe it.

Do this, in this order

1. Get everyone out of the house — people and pets. Don't stop to open windows or hunt for the source.
2. Call 911 from outside (or your fire department's non-emergency line if you're sure nobody has symptoms — but 911 is the right default).
3. Don't go back in until the fire department has checked the air.
4. If anyone has a headache, dizziness, nausea, or confusion, say so on the call. Those are CO symptoms; they get worse the longer you stay inside.

Alarm vs chirp

• A loud, continuous or patterned alarm (usually four beeps, pause, repeat) means CO detected. Leave.
• A single chirp every 30–60 seconds means low battery or the detector has reached end of life (most last 5–10 years and chirp when they expire). Not a CO alarm — replace the battery or the unit.
• A steady beep with no pattern, or an "ERR" on the display — read the label on the back; it's usually a fault code, not CO.

When in doubt, treat it as CO and leave. A false alarm costs you fifteen minutes on the lawn. The other kind costs more.

Why the furnace is the first suspect

Any appliance that burns fuel makes CO: furnace, boiler, water heater, gas range, fireplace, a car in an attached garage, a generator run anywhere near the house. A furnace with a cracked heat exchanger or a blocked flue is the classic cause of a winter CO alarm — and it can run apparently "fine" while doing it.

That's why the fire department checks, not you. And it's why a furnace that set off a CO alarm should be inspected by a technician before it runs again, even if the house tests clear afterward.

After you're cleared

• Have the furnace (and water heater, if gas) inspected before using them.
• Replace any detector older than 7 years, and put one on every level of the house, near the bedrooms.
• Never run a generator, grill, or car in an attached garage, even with the door open.

If the furnace is inspected and cleared, and your only remaining problem is that it doesn't heat well, then the normal troubleshooting applies.

Related pages

• Smell gas near the furnace
• Burning smell when the heat comes on
• Furnace won't turn on

Start the free troubleshooter → — only after the house has been checked and cleared. Free. No sign-up. Don't pay $150 to flip a switch.

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