I'm learning about catalytic converters, and learned that a car with a functioning converter doesn't output carbon monoxide (CO) because it converts it into carbon dioxide (CO2) and water (H2O). How exactly does someone die of CO poisoning if they leave the car running, if the converter ensures this doesn't happen? Is there a step I'm missing?

For example this article states:

Wanda Brown Steele, Taylor's mother, said believes Taylor died of carbon monoxide poisoning. "The car was running, and the snow was still coming, so it blocked the pipes, the exhaust pipe."

This old movie also claims you could die of CO poisoning by running your car in the garage. Shouldn't the catalytic converter prevent this from happening?

  • An older vehicle without a cat? In UK, cats became mandatory fitting in 1993, in USA I think 1975. Another possibility is the cat was removed. Plenty of people do that. Yet another, is that the exhaust is leaking into the cabin from between the engine and the cat, and if the tail pipe was blocked, quite possible. Commented May 14, 2023 at 21:59

2 Answers 2


A catalytic converter isn't perfect. It will convert most of the CO over, but it isn't going to get all of it. A certain percentage will still remain, even in a highly efficient converter. This website states:

Directing exhaust fumes through a metal housing coated with the catalyst can remove up to 98% of pollutants ...

It does say "up to". That means some of the CO is going to remain. It will take longer for the CO levels to become toxic in an enclosed space, but have no fear, sooner or later there would be enough to kill you.

Secondarily, something to remember is CO2 can kill you as well. It just does it differently. If your body can not get enough oxygen, you're still going to be dead.


Because you're talking about an extreme edge case.

The news articles I read stated that the carbon monoxide poisoning was a theory postulated by a member of the family i.e. bereaved, with no other source attributed. That person is not indicated to have any medical or technical qualifications to make such an assessment.

That said, it's not a terrible theory IMO. The snow got pretty high, and it's likely that it covered up the car's normal air inputs. However there was still void space under the car, particularly as the exhaust pipe melted snow there. The engine would tend to draw air from that under-space as its normal inputs got covered by snow. Likewise the exhaust could still discharge into this under-the-car space.

Especially if they plowed into a snowbank (covering it on front and sides) and then got a couple inches more snow, the only route for engine air may have been from the rear down that under-car tunnel.

So you'd have an edge condition where the car is operating in a confined space with poor air circulation; CO2 isn't being carried away and fresh oxygen isn't being replaced. Exhaust is very close to 0% oxygen if things are working properly, all replaced with CO2. While other air is being mixed in, the air is becoming richer and richer in CO2 and depleted in oxygen.

Cars are designed to be run outdoors where we can count on ~21% oxygen and a tiny fraction of 1% of CO2. The system is built to assume normal atmosphere blend; cars do not have HC, CO and NoX sensors in the exhaust stream; hence the semiannual ritual of smog checks :) Their only sensor is an oxygen sensor that tells them more or less digitally, "lean" or "not". So cars rely utterly on the assumption that atmospheric mix is near normal. The car would have no way of knowing the atmosphere was corrupted and thus, its mix ratios were incorrect.

  • I understand your reasoning. But CO2 inhalation will wake you up, whereas CO (monoxide) will not wake you up. So I'm guessing there was either another mechanism in action, or the family member could've been speculating wrongly.
    – M -
    Commented May 16, 2023 at 5:51
  • @Marquizzo I agree the family member may be incorrect. But anywhere you're getting CO you're also getting CO2 in large quantities. I also thought CO2 wakes you up and fills you with a sense of panic/gotta get out of here/get fresh air, but now I can't find research to support that. Commented May 16, 2023 at 20:56

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