The CO Alarm End-of-Life Signal and How Long a Sensor Lasts

Why this matters

A CO alarm that chirps after a fresh battery is not broken. It is telling the household that its sensor can no longer be trusted to detect carbon monoxide, and it is designed to keep saying so until someone replaces it. The usual response in a house is to silence it, swap the battery again, or take it down "until we get a new one." Each of those leaves the family with a device on the wall that looks like protection and is not. A tech who can tell an end-of-life signal from a low battery, and who says plainly that the answer is a new alarm, has closed the gap in two minutes.

First, make sure it is a chirp and not an alarm

A CO alarm sounding its alarm pattern is an emergency, not a maintenance call. Household CO alarms use a distinct alarm pattern under NFPA 72, as adopted: four short beeps, a pause, repeating (smoke alarms use three). If the display shows a CO reading without the alarm sounding, open windows, shut off fuel-burning appliances, and get the appliance checked; if the number climbs, treat it as an alarm. If the alarm pattern is sounding, everyone leaves the house now, leaves the door open behind them if it is on the way out, and calls 911 or the fire department from outside. Anyone with headache, dizziness, nausea or confusion gets medical attention. Nobody stays in to look for the cause. Real-alarm response is its own card; see related: Carbon Monoxide Testing + Response Reference and How to Respond When a CO Alarm Sounds and You Are Not the Heating Tech.

What this card covers is the other sound: a short chirp or burst of chirps with long silence between, which is the alarm asking for maintenance.

Why a CO sensor wears out

Most household CO alarms today use an electrochemical sensor: electrodes in an electrolyte, inside a small cell. Carbon monoxide that reaches the sensing electrode reacts there and produces a tiny current that rises with the CO concentration. The alarm's electronics integrate that current over time and sound when the dose crosses its threshold.

The cell is consumable. Over years the electrolyte dries out, the electrodes lose activity, and exposure to some household vapors can contaminate them. The result is that the sensor produces less current for the same CO, so the alarm reads low and responds late or not at all. The electronics cannot fully compensate, and a sensor test button does not check it: the test button exercises the horn and circuit, not the cell's response to real CO.

That is why the maker states a service life and why the alarm counts down to it. Stated lives on current units commonly run 7 to 10 years; the label and the manual are the authority. Older units with other sensing methods may have shorter lives.

What the alarm is supposed to do at end of life

Listed alarms are tested to UL 2034, which also sets how a CO alarm must respond while it is healthy. Under UL 2034, an alarm must sound within 60 to 240 minutes at 70 ppm, within 10 to 50 minutes at 150 ppm, and within 4 to 15 minutes at 400 ppm, and must not alarm at 30 ppm over 30 days. Those are dose curves, not instant thresholds, which is why a working alarm may stay quiet through a brief low-level whiff. It is also why a degraded sensor is dangerous: an alarm whose sensor reads half of the true CO behaves, at 150 ppm, roughly like a healthy alarm seeing 75 ppm, so its response stretches from the 10 to 50 minutes a healthy alarm is allowed at 150 ppm toward the 60 to 240 minutes allowed at 70 ppm.

At the end of its service life, a current listed alarm gives an end-of-life signal. The form is set by the maker: a distinct chirp pattern, often a burst of several chirps repeating at an interval, or a word or code on a display, or a light pattern. It differs from the low-battery chirp, usually a single chirp about once a minute. The label on the back or the front of the unit decodes the patterns, and reading it is the whole diagnosis.

Many alarms let the end-of-life signal be silenced for a limited period, sometimes a limited number of times, so the household can get through a night and buy a replacement. The manual states both. That feature is a grace period, not a reset, and the alarm is designed to resume.

Low battery, end of life or fault

The test that separates the first row from the second: put in a known fresh battery of the type the label specifies. If the chirp stops and stays stopped, it was the battery. If it returns, or the pattern matches the end-of-life description, it is the sensor's life, and no battery will fix it.

Worked example: the chirp that came back twice

The call. A customer phones the office in October 2026: the hallway CO alarm keeps chirping and they have changed its batteries twice, once in August and once in September. The office books it as a battery change. A plumber already scheduled for a faucet repair that afternoon is asked to look.

Safety first. On arrival the plumber listens before anything else: a burst of chirps every so often, long silence between, no four-beep alarm pattern, no CO reading on the display. Nobody in the house reports symptoms. It is a maintenance signal, so it can be diagnosed.

Read the label. Plug-in unit with battery backup, listed to UL 2034. Manufactured May 2016. The label states a 10-year life and decodes the patterns: a single chirp every minute is low battery; a burst of five chirps repeating is end of life. The plumber times the burst: five chirps, then about half a minute of silence, repeating. End of life.

Check it against the date. May 2016 plus 10 years is May 2026. October 2026 is 5 months past. The signal is behaving exactly as designed; it has been running since around the spring.

What the customer had been doing. The two battery changes did nothing, because the battery was never the problem. The customer had also pressed the silence button "a few times" over the past week, three by their count. Each press quieted it for a while and then it resumed, which is the manual's limited grace period being used up.

What gets done. The plumber tells the customer: this is the alarm saying its sensor is worn out, a new battery will not stop it, and until it is replaced the hall has no working CO alarm. The customer has a new plug-in unit in the garage from a warehouse-store multipack. Its label reads: manufactured August 2026, 10-year stated life. The plumber plugs it into the same hall outlet (outside the bedrooms, which is the required location; see related: Where a House Needs CO Alarms Under the 2021 IRC), fits the backup battery, waits for the power-up sequence to finish and presses test: four beeps, pause, repeating, then silence. On the job note: old unit, mfg 2016-05, end of life confirmed by pattern; new unit, mfg 2026-08, replace by 2036-08. Old unit's batteries removed and the unit bagged for disposal as its label directs.

Time. About 15 minutes inside a faucet call. The office changes its intake script so that "chirping after a new battery" is booked as "likely end of life, bring a replacement," not as a battery change.

What changes the answer.

• The burst had been a single chirp once a minute that stopped with a new battery. Low battery, and the visit is a battery change plus a date check.
• The unit was a combination smoke and CO alarm. End of life on one side retires the whole unit, and the replacement has to be compatible with any interconnected smoke alarms (see related: Hardwired, Interconnected and Battery Backup: What Each Word Means).
• The unit was hardwired. Replacing it on its harness, or rewiring it, is electrician work wherever the state or locality licenses it, as many do; the plumber writes it up instead of swapping it.
• The alarm's pattern did not match anything on the label. Treat it as a fault and replace it. An alarm that cannot explain itself is not one to keep.

Get ahead of the signal

The end-of-life signal is the last warning, not the first. The replace-by date is printed on the unit from the day it is made, so a shop that records it can replace the alarm before it ever chirps. A reasonable default: when a tech records a CO alarm's replace-by date, the office sets a reminder for about 60 days before it, which gives the customer time to buy a unit and book it into a visit they already need. In the example above, a reminder set from the 2016 unit's label would have fired in March 2026, two months before the stated end of life and seven months before the customer called, and before either wasted battery change. For the new unit, the same rule puts the reminder at June 2036.

Why silencing it again is not a fix

Every silence press on an end-of-life alarm trades a night of quiet for a night of no protection, and the alarm is built to stop allowing it. Removing the batteries from a plug-in unit to stop the chirp is the same act with less honesty; many units chirp anyway on AC, or go quiet and stay dead. If a customer will not replace it today, the job note says the alarm is at end of life, the date, and that the customer was told it no longer detects CO reliably. That is the stance this shelf takes on every life-safety device: working, or the customer told in writing.

References

• UL 2034, Standard for Single and Multiple Station Carbon Monoxide Alarms: alarm response criteria and end-of-life signaling
• NFPA 72, in the edition your jurisdiction adopts, for household CO alarm replacement and signal patterns
• CO alarm manufacturer instructions, for stated service life and the meaning of each chirp and display code
• See related: Carbon Monoxide Testing + Response Reference; Reading the Manufacture Date and the Ten-Year Replacement Rule; Where a House Needs CO Alarms Under the 2021 IRC