My experience with built in detectors is less than even fair. My original built in Propane/CO detector failed after 2 1/2 years. I full time so it was working the whole time. Fortunately it started going off at 3:00am a couple of days in row. I replaced it with a new one and I also have a separate battery operated unit in the bedroom.
After I replaced the built in unit I took the old unit, hooked it up to a battery and exposed it to propane. It did not work. Remember it was 2 1/2 years old from the date of manufacture.