Why is my carbon monoxide alarm beeping, and when is it an emergency?

Four beeps, then silence, then four more beeps. On the two biggest residential brands, that is the one pattern that means carbon monoxide has been detected. Anything slower and quieter is the alarm asking you for maintenance.

Count before you do anything else. The emergency pattern and the maintenance chirps call for opposite responses. Mixing them up can mean pulling the battery out of an alarm that's working, or standing in the hallway reading a manual while the CO level keeps climbing.

The patterns, side by side

Kidde and First Alert both publish their codes. Most of the common patterns match across the two brands, but the details vary by model, and the label on the back of the alarm takes priority over anything general, including this table.

Neither brand treats the chirp patterns as a CO detection. The four-beep pattern is one, and First Alert's instruction for it is to get outside and call 9-1-1.

If it's the four-beep pattern

Leave first. Don't diagnose anything from inside the house. EPA's guidance for a suspected exposure is to get to fresh air right away, turn off combustion appliances if you can do that on the way out, and get medical attention if anyone feels unwell. Call from the driveway, not the kitchen.

Don't decide it's a false alarm because nobody feels sick. CO has no color and no smell, and EPA notes that its symptoms (headache, dizziness, nausea, confusion) can look like the flu. One clue EPA points to: symptoms that ease when you leave the house and come back when you return.

Here's the single most useful thing to do before you go, if the alarm has a digital display. Don't press reset. Kidde's peak-level-memory models, like the KN-COPP-B-LPM, store the highest CO reading since the last reset, anywhere from 11 to 999 ppm. Pressing reset clears that number. The responder checking your furnace will want it.

Why the alarm can't sound at low levels

A UL 2034-listed alarm is built to go off before CO reaches a level that stops you from reacting. It isn't built to report every trace of CO. Kidde's 2026 spec sheets list the alarm windows the standard requires:

For comparison, EPA's outdoor air standard is 9 ppm averaged over 8 hours and 35 ppm over 1 hour, and there's no agreed indoor standard. So by the time a 70 ppm alarm sounds, the air may have been well above those outdoor limits for an hour or more. Treat the alarm as real even after the beeping stops.

If it's a chirp

A single chirp about once a minute on a replaceable-battery model is the alarm asking for a new battery. Replace it with the type printed on the back, then hold the test button until it sounds.

A chirp every 30 seconds is different. Kidde says that pattern starts when the alarm reaches the end of its rated life, and that replacing the batteries won't stop it, because the sensor is done. Kidde is also explicit that an alarm in end-of-life mode doesn't detect carbon monoxide. Some models let you hush the end-of-life chirp for three days at a time, for up to 30 days. That hush is meant to give you time to buy a replacement. It doesn't extend the sensor's life.

Here's the flat rule. If the alarm is past the life printed on it, replace it, even if the test button still makes it beep. The test button checks the horn and the circuit board. It doesn't check whether the sensor can still detect CO.

How long is that life? It depends on the brand and the year the alarm was made. Kidde says its CO alarms made since 2013 are rated for 10 years, with listed exceptions. First Alert says most CO alarms last 5 to 7 years and recommends replacing any of them after 7 years. Check the manufacture date on the back. If you can't read it, assume the alarm is due.

A 10-year sealed-battery alarm such as First Alert's CO710 often lists between about $26 and $49 at retail, depending on the store and whether it's on sale. Buy one listed to UL 2034, which is the certification EPA tells you to look for.

Where the CO came from

Once the fire department or utility has cleared the house, the alarm has done its job, and the question becomes which appliance produced the CO. EPA's list of the usual sources is short:

• Furnaces and chimneys that leak or backdraft, and water heaters, wood stoves, and fireplaces that backdraft.
• Furnaces and boilers that are worn or poorly adjusted, or whose flue is the wrong size, blocked, disconnected, or leaking.
• Gas stoves and unvented gas or kerosene space heaters.
• Car exhaust from an attached garage, and portable generators run inside the house, garage, crawlspace, or a shed.

The generator and the car are the two causes you can fix yourself today: keep them outside and away from openings. Everything else on that list is gas appliance work, and none of it is DIY. Leave the appliance off and get a licensed HVAC or gas technician to inspect the venting and combustion before anyone relights it.

The finding you're most afraid of is a cracked heat exchanger. That's the metal wall inside a furnace that keeps combustion gases out of the air going through your ducts. Trane's position is that a cracked heat exchanger shouldn't be repaired. It should be replaced, or the whole furnace should be. If the furnace is old enough that you're already thinking about replacing it, ask for a written replacement quote before you approve a new heat exchanger. Also ask the technician to show you the crack, either with a camera or with combustion readings. Our HVAC quote guide covers what that replacement quote should include.

Make sure you have the right number of alarms

When you buy the replacement, check the count. The IRC's carbon monoxide section, R315, requires CO alarms in any home with a fuel-fired appliance or an attached garage with an opening into the house. The alarms have to be outside each separate sleeping area, in the immediate vicinity of the bedrooms. If a fuel-burning appliance is in a bedroom or its attached bathroom, there has to be an alarm inside that bedroom too. A permitted remodel can trigger the same requirement in an older house. Your city or state may have amended the code, so check with the local building department.

Then put two things on your calendar: the alarm test interval from its manual, and a fall heating check. ENERGY STAR recommends having a contractor do a pre-season check-up every year, with heating checked in the fall. That check covers the gas connections, burner combustion, and heat exchanger. An alarm only tells you CO is already in the house. The fall check is how you catch the problem before that.

Research sources

• Kidde: Why Is My Carbon Monoxide Alarm Beeping or Chirping?
• Kidde: CO alarm replacement FAQs
• Kidde: KN-COPP-B-LPM CO alarm with peak level memory
• Kidde: CO alarm spec sheet (21033773, Rev 2026)
• First Alert: Why Is My Carbon Monoxide Detector Beeping?
• First Alert Canada: What Do the Beeps From Your Carbon Monoxide Alarm Indicate?
• EPA: Carbon Monoxide's Impact on Indoor Air Quality
• EPA: What About Carbon Monoxide Detectors?
• ICC: IRC Section R315, Carbon Monoxide Alarms
• ENERGY STAR: Maintenance Checklist
• Trane: Cracked Heat Exchanger