Carbon Monoxide in garage
The answer is fairly obvious to me. For one thing, 55 parts per million isn't "high". 


  When cold engines first start, they run rich," Greiner said. The catalytic converter is cold and not converting deadly carbon monoxide (CO) to carbon dioxide (CO2). Concentrations in the exhaust can be more than 80,000 parts per million. Concentrations so large fill the garage with carbon monoxide in a very short time even with the door open. ...after only two minutes of warm-up in the opened garage, carbon monoxide concentrations rose to a lethal 575 parts per million


https://www.extension.iastate.edu/pages/communications/CO/co_car.html