Plus 3 on don't use CO2
The body has excellent defences against elevated CO2, it gives great discomfort and a feeling of absolute panic and terror (I go caving, and I've been in areas with elevated CO2, it is not good, try holding your breath for as long as you can, it is the rising CO2 in your blood that causes the discomfort and urge to breath).
As Bill D says, the body has no defence against depleted O2, so a nitrogen or inert gas atmosphere will result in lost conciousness and death. Actual death takes several minutes after breathing ceases, if you introduce oxygen before that, the poor thing will probably gasp and begin breathing again.
I know labs and some slaughter plants use CO2, this is a hang over from the days of having coal gas containing CO available, it is also based on DesCarte's mistaken theory, that as animals can't speak, they can't feel pain (surgeons use the same crap to justify surgery on newborn babies without wasting their money on pain relief
).
(Edit: this side of the Atlantic vets receive NO TRAINING IN ANIMAL WELFARE, and many are actually taught that "animals do not feel pain", a few might actually believe it)
Using engine exhaust gas may have worked well in the days of carburettors and chokes on gasoline engines, but there isn't much CO in today's car exhaust fumes.
There are other ways of getting CO, which I'll share through PM, but not in open forum - I know a couple of people who wouldn't be here today if they'd known when they were suffering depression - They both tried with car exhaust, and both said that the next ten days it felt like they'd gone 10 rounds in the ring with Mike Tyson.