Ventless heaters are illegal in Ca., probably because of the diverse cultural makeup. They're good for buildings that are not air tight and will vent through drafts. Problems arise when there are 2 stories involved, the heated air will go up while the cold air pushes down. Sensors in the heater will not detect low oxygen until it may be too late. For a garage, you should be ok. In a home, a detector should be placed in the highest habitable area.
"When I was younger, I could remember anything, whether it had happened or not." - Mark Twain