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Burning plastic smell from catalytic converter

Where do I start. I have a 2001 Nissan Sentra. 128,000 miles. In Aug. of last year, the engine detonated, so I had it replaced along with other parts. Shortly after that, I noticed a burning plastic smell coming from the car after driving it for a while. I confirmed that that the smell wasn’t coming from a plastic bag. Fast forward to Dec 2011, my check engine light comes on. I go to Autozone, have the code read; it’s the dreaded 420 code. I go to have it checked out, was told my cat went bad and probably because of the old engine and that the smell is probably coming from that. The check engine light went off on Christmas day. Fast forward to last month, the light comes on again and I pony up the cash to have the cat replaced since inspection is coming up in June. Now, I had an engine diagnostic run right before I got the cat replaced and it came up fine. The cat was replaced on 4/20. Everything seemed fine. The burning plastic smell went a way, but lo and behold, it came back this week. I don’t know what to do. I did a little search on the net and it seems that burning plastic smell indicates there is something going on with engine and possible misfire. I can’t take this anymore! I’ve already put so much money into this stupid car, but I need it, buying another one is not an option right now.

if there’s a misfire, you should get a flashing check engine light.

New cats smell bad for awhile until they’re broken in, which can take awhile. Some describe it as burning plastic, but most find it smells like rotten eggs or sulfur. I wouldn’t worry too much about it unless you get a check engine light or the smell continues beyond a few weeks.

New exhaust parts smell like burning plastic as the machine oils (and sometimes labels) cook off. Catalytic converters are particularly prone to do so because they can get over 800F and because in new vehicles they’re in or directly connected to the enhaust manifold, an area where the smell is likely to be noticed by the occupants.

The rotten egg smell if you have it is actually caused by sulpher in the fuel binding to and being carrried by oxygen in the the form of sulpher dioxide or with nitreous oxides in the form of H2SO4 (how the heck do you do those small subtext letters?). It’s the sulpher you smell. It’s associated with cat converters because the nitreous oxides that the converter is designed to split up can actually bond with the sulphers instead, and/or the oxygen that’s stripped from the NO2 molecules can also bond with the sulpher to form the sulpher dioxide. In short, it’s a good “breeding” environment for the smelly sulpher compounds.