A method of operating a fuel cell stack that powers a vehicle includes determining when the vehicle is in a non-moving state, calculating an O2 concentration over time of an assumed enclosed space while the vehicle is in the non-moving state, establishing a set of O2 concentration concern levels that includes a first O2 concentration concern level being less than a standard atmospheric O2 concentration and a second O2 concentration concern level being less than the first O2 concentration concern level, comparing the O2 concentration of the assumed enclosed space over time with the set of O2 concentration concern levels, and operating the fuel cell stack without restriction when the vehicle is in the non-moving state so long as the O2 concentration of the assumed enclosed space remains greater than the first O2 concentration concern level.