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.