This is a Honeywell 5898 that is ~6 years old. nothing has changed in the environment, its in a hallway so no windows ect, I checked and no bugs. (hadnt thought of that)
I have a backup sensor I am installing in same location as orig and made it same as orig zone # for CO.
I changed orig sensor to 13 def and zone RC to 00 and setup rule to email me if it gets confused.
If I get false alarm, it will tell me if sensor or M1G and I will report