What I know is that they clip the vent hose to the CC then the machine pump air in the tank to see if the vent leaks vapor to the atmosphere, then it would lose the pressure at the pump. I asked the tech, if the hose and the CC were inaccessabile as some european cars have (And I think our car too). How would you do the test? he said he would skip it. So that's the luck we might have.