CO Detector

I used the Flight Data Systems one and am happy with it. It's easy to install and integrates well with the G3X. I have the CO level displayed as one of the data fields on top of my copilot display.
 
I also used the FDS unit, good features, good price, and the biggie for me was the sensor life, and replacement cost and simplicity.

I suggest anyone looking into CO monitors consider the fact the sensor is time limited and will need to be replaced at some time.