The Kidde KN-COB-B is inexpensive because it lacks a digital display. Bear in mind, though, that most experts prefer CO detectors with digital readouts, which continually display carbon monoxide gas levels in the home and can alert you to borderline levels that may not yet be dangerous. Still, the Kidde KN-COB-B has a green LED that flashes every 30 seconds to indicate that the unit is working properly, a test/reset button and multiple mounting options.
We read several owner-written reviews of the Kidde KN-COB-B at Amazon.com.
There are only about 10 user reviews here for the basic Kidde Battery Operated Carbon Monoxide Alarm KN-COB-B, but most are positive. One owner reports false alarms.
Review: Kidde KN-COB-B Basic Carbon Monoxide Alarm with Electrochemical Sensor, Contributors to Amazon.com