Carbon monoxide alarms measure CO levels in the air. CO can be caused by faulty heating equipment, particularly flues. Combustible materials use oxygen to burn and produce carbon monoxide as a by-product. Carbon monoxide is a lethal--and, unfortunately, odorless--gas. A carbon monoxide detector will sound an alarm if it detects CO gas nearby. To keep your CO detector functioning properly make sure its batteries are fresh.
Slide the First Alert CO detector off the wall when the device chirps roughly every minute--that's the low battery warning. Wall-mounted detectors are simply hooked onto some exposed screws.
Flip open the battery door.
Remove the used batteries and insert two new AA batteries. (Make sure the polarity is correct when inserting the batteries: the plus and minus signs on the batteries should be aligned with the plus and minus signs on the detector.) The detector will produce a little chirp if you've got it right.
Close the battery compartment.
Slide the device back onto its exposed screws by aligning the eyelets under the detector with the screws in the wall.
Press and hold the test button until the LED flashes. The detector should then sound four beeps followed by a pause and another four beeps. The red light will flash. This indicates the device is working.