A carbon monoxide detector, like a smoke detector, is designed to provide warning when dangerous gases are present in the home. Carbon monoxide detectors that are battery-powered or have a backup battery include a built-in warning if the power is running low. You can tell if the battery is running low on power and should be replaced by the warning sound it makes. All that is needed is the instruction manual that came with the carbon monoxide detector and a tool to replace the battery with a new one.
Listen to the "chirping" sound being made by the carbon monoxide detector every few seconds. Check the instruction manual that comes with the detector to verify that this is a low-battery warning that indicates the battery inside the detector is nearly depleted.
Remove the front cover of the carbon monoxide detector -- this may involve twisting the cover counterclockwise or removing screws around it using a Phillips screwdriver.
Remove the battery from inside the battery compartment -- if the battery is attached to a battery clip, remove the clip first. Dispose of the battery properly.
Insert a replacement battery that is compatible with the detector in the battery compartment -- if a battery clip is in use, attach the contacts on the battery clip to the appropriate contacts on the battery first.
Place the cover back on the detector -- this involves either twisting it on clockwise or reattaching screws around it using a Phillips screwdriver.
Press the "Test" button on the side of the detector and hold it in until the horn sounds. Release the button. Listen to hear that the "chirping" is no longer sounding -- indicating that the replacement battery has sufficient power to deactivate the low battery power alarm circuitry in the detector.
In rare cases, there may be a battery tray that pulls out of the detector, leaving the cover intact.