CO2 Detectors are a must for every garage. Here are some good examples
CO2 Fresh Air Indicator and Alarm for OSHA
[ReviewAZON asin="B0043G8L8K" display="fullpost"]
Indoor Air Quality Meter - CO2, Temperature, Realitive Humidity
[ReviewAZON asin="B001PDGFR8" display="fullpost"]
General Tools CDM77232 High Accuracy Temperature, Humidity and Carbon Dioxide Monitor
[ReviewAZON asin="B0035BL1RW" display="fullpost"]