Plastic bag melted onto catalytic converter smells just as you'd expect: disgusting.

Dear Car Talk | Dec 01, 2002
Dear Tom and Ray:
RAY: Gee, Harry. Something recently happened to change MY car's smell for the worse, too. My brother got in.
TOM: This is a common scourge, Harry. We see it mostly with plastic supermarket bags. They get under cars and instantly melt all over the red-hot catalytic converter.
RAY: It won't do any real harm to the car, and eventually, it'll burn off altogether. But it'll take weeks. So if you want to accelerate the process, you can get one of those scrapers that uses razor blades and have a go at it. We also find that a wire brush helps. Make sure you do it when the exhaust system is cool, so you don't burn yourself. But with those two tools, you should be able to get 95 percent of it off, and the rest will burn off by itself.
TOM: And who knows, you might actually get to like the smell! Whenever I get in his car, my brother now looks for plastic bags to run over.
RAY: Gee, Harry. Something recently happened to change MY car's smell for the worse, too. My brother got in.
TOM: This is a common scourge, Harry. We see it mostly with plastic supermarket bags. They get under cars and instantly melt all over the red-hot catalytic converter.
RAY: It won't do any real harm to the car, and eventually, it'll burn off altogether. But it'll take weeks. So if you want to accelerate the process, you can get one of those scrapers that uses razor blades and have a go at it. We also find that a wire brush helps. Make sure you do it when the exhaust system is cool, so you don't burn yourself. But with those two tools, you should be able to get 95 percent of it off, and the rest will burn off by itself.
TOM: And who knows, you might actually get to like the smell! Whenever I get in his car, my brother now looks for plastic bags to run over.
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