Carbon monoxide (CO) detectors, like smoke detectors, have a test function. Test your CO detector on a regular basis. For instance, Kidde, a manufacturer of CO detectors, recommends testing a detector on a weekly basis. Some regular maintenance also helps keep a detector functioning correctly.
Verify that the green power light on your detector is on. It should flash every 30 seconds to indicate the unit is monitoring for CO.
Press the "Test/Reset" button. On a unit without a digital display, if the device is working correctly, you should hear four short beeps followed by a five-second pause, followed by another four quick beeps. On the Kidde KN-COPP-B (which has a digital display) you will see three eights and then a number, usually around 200. The red light emitting diode (LED) will flash along with the beeps.
Wait a few seconds and the unit will start monitoring for CO again.
Check the lights periodically. If the red LED flashes every 30 seconds, replace the batteries. If the red light remains on, the unit has failed. Replace it.
Change the batteries. Change them at least once a year in any case. After changing the batteries, reinstall the alarm and test it. Check that the green light is flashing every 30 seconds and make sure the red LED isn't flashing every 30 seconds. If it is flashing red every 30 seconds and you've replaced the batteries, the detector has failed, and you should replace it.