This study measured exhaust carbon monoxide of two automobiles - one without and one with catalytic converter - idling in a closed garage as well as the carbon monoxide and oxygen concentrations within the garage. As anticipated, the declining oxygen levels of the test “tricked” the carbureted car without catalytic converter into richening the mixture and thus producing dangerously high exhaust CO levels. Although the second car with a catalytic converter and on-board oxygen sensor control system initially produced low CO exhaust levels, it also eventually was “tricked” into producing high levels of CO. Thus the potential exists for a “clean automobile” to cause lethal levels of carbon monoxide to accumulate in a closed garage.In order to predict the amount of time for lethal levels to be reached, this paper develops a differential equation model which can be solved on a computer to successfully predict oxygen and carbon monoxide concentrations in a garage as functions of time. The model takes into account estimated engine volumetric efficiency, measured exhaust CO production, and leakage through garage walls and under the door. The model assumes that leakage rate is equal to concentration difference divided by an empirically determined time constant.