I've got a caravan type CO detector in my ST87. Screams loudly enough to be heard above the engine at climb power, gets through the noise canceling headset no problem. Has a shut off button and runs on batteries. No, of course it isn't certificated. Occasional flashing light to show its working and ready to detect so we put it behind the pilot. It's easy to reach to cancel the alarm. Fairly quiet beeps at a very low level of CO, then faster and louder if the situation gets serious. I've only heard it on test mode so far, long may that continue. I've never had any confidence in a faded bit of cardboard.