A CO detector is a decent thought IMHO, but it might go off, depending on the vehicle, fairly often, which would negate that 'goodness'. If the garage is attached, a CO detector with smoke detectors in adjacent areas is good.
Nest's 'Protect' devices don't need the trigger line...they use their own wireless network to communicate, and also report to the internet via WiFi. When one goes off, it annouces through all of them where and what's happening. Not much more than combining a CO and smoke detector's cost...it will also send you an email if one goes off, so it's good for remote monitoring as well.
Keep in mind that smoke detectors don't last forever while retaining their full sensitivity...the recommended replacement interval is 10-years.