How to Test a Carbon Monoxide Detector (Real-CO Method)

Before you test anything, walk through and write down every CO detector you find. NFPA 720 requires a CO alarm outside every sleeping area (the hallway between bedrooms is the standard spot) and on every level of the home - basement included, especially near the furnace or water heater. Many newer detectors are combination smoke + CO units; both functions need to be tested separately.

If a level or hallway has no detector at all, add it to a shopping list. CO is heavier than warm air but distributes evenly through a room, so the alarm does not need to be high or low - mount per the manufacturer instructions, usually on the wall or ceiling.

Climb up to each detector and push the round TEST button on the front. Hold it for about 5 seconds. The alarm should sound a loud four-beep CO pattern (beep-beep-beep-beep, pause, repeat) at around 85 decibels at 10 feet - loud enough to wake you from sleep. The green power LED should be solid before you start the test.

This press-and-hold test only confirms two things: the speaker works and the battery has juice. It does not test the CO sensor itself. A detector with a clogged or expired sensor will still chirp on the button press and then completely miss real CO in the air. That is why step 4 below matters.

The button test is monthly. The real test is yearly, and you need actual carbon monoxide to do it. The pro standard is a Solo C6 canister (about $40, used by professional fire alarm inspectors) which dispenses a calibrated dose of CO. Consumer-grade canisters from First Alert and Kidde do the same thing for less, and the small cans last for many tests.

If you do not want to buy a can, an electrochemical low-level CO monitor (around $130) lets you test by measuring background CO levels in real time - useful if you live with a furnace, attached garage, or gas appliance and want to catch slow leaks before any alarm trips. Both options live in the affiliate links below.

Take the detectors down and move to a garage or open basement with the door propped open. Never run this test in a closed bedroom or near sleeping family members - you are intentionally releasing a small amount of poison gas. Open a window in addition to the door if you can.

The professional method (shown in the video) uses a clear plastic bag to contain the CO around the detectors so the gas concentrates at the sensor instead of dissipating. Lay each detector face up inside the bag, drain the air out, then dispense the Solo C6 into the bag and seal it loosely. For consumer cans without a bag, spray a 1 to 2 second burst directly at the vent slots from about 2 inches away.

Start a stopwatch on your phone the second you dispense the gas. A working sensor must alarm within the UL 2034 spec: between 5 and 15 minutes at 70 to 200 ppm. Faster than 5 minutes at low concentrations is actually a fail (the unit is too twitchy and will nuisance-trip), and slower than 15 minutes is a sensor fail.

A First Alert unit with the LCD readout shows the ppm climbing in real time - in the video it reads 90 ppm at 3 minutes 33 seconds, then climbs to 250+ ppm before most units finally alarm. If you do not have an LCD detector to read ppm, just trust the timer.

Listen for the alarm pattern. CO detectors sound a four-beep pattern (beep-beep-beep-beep, pause, repeat) which is different from the three-beep smoke alarm pattern. Units with voice annunciation like the Nest Protect will say 'Emergency, carbon monoxide' and glow red. Note which detectors fired and how long each took.

Any unit that did not alarm within 15 minutes is failed. Mark it for replacement and pull it down. A detector that passes the TEST button but fails real CO is the exact kind that lets a real leak kill you in your sleep - the speaker works, the battery works, the sensor is dead.

Pop the door wide and open every window before you do anything else. Walk away for 20 to 30 minutes. The CO from the test will dissipate fast in a ventilated space but you do not want to be standing over it while it does. Do not silence a still-alarming detector by pulling the battery - let it finish, then carry the detector outside to clear it.

For any detector that failed the real-CO test, replace the whole unit (do not just swap the battery - the sensor is the dead part). For any detector older than 5 to 10 years per the date stamped on the back, replace it even if it passed. CO sensors are electrochemical and degrade with time even when the unit is unplugged. Write the new install date on the side of the replacement with a Sharpie so year 10 is easy to spot.