In a private e-mail to another forum member I wrote the following,
"I like the American Sensors product line. I like them because they use what's known as a three terminal sensor. Any CO detector is only as good as the sensor element that measures the gas concentration. The detector element has an electrolyte, a chemical that changes when gas molecules are present. This chemical evaporates with time and the sensor becomes less sensitive over time. The American Sensor brand sensor has a way with the third terminal to sense the deterioration and compensate for it. Others brands may have this also but I haven't been able to get any technical details to verify it. Over years (5-7) of use all brands American Sensors included need to be replaced. Expensive units used by fire departments have replaceable sensors. The home market units do not. The entire unit must be replaced."
Please note this is a technical opinion of the basic CO detector sensor element, not the overall unit. All CO detectors for sale will have some kind of CO detector sensor plus additional electronics, microprocessor, display, alarm, etc. integrated with the sensor. It's not easy to find out for a specific product what the basic sensor is. The American Sensors web site is not very good and it's unclear if the same product is sold under another name in the USA. I don't have time to research a source. I encourage some forum member to do so and post the results.
If someone is interested in the sensor technical details, data sheets, electronic schematics, etc. send me a private e-mail.