The first thing that comes to mind when a car is started in a garage is carbon monoxide (CO). Today's cars with catalysis emit higher levels of CO₂, which is why we strongly recommend multiple sensors.

If a CO sensor is controlling the air flow, you will find that the exchange rate is far too low. When the parking lot maintenance technician realises that, they will switch to permanent ventilation and no energy is saved. We recommend control with CO₂ and alarm with CO. 

For a deeper look into the subject, click here to read an article by Hans Martin.