Have you added oil recently and left the cap off perhaps?
Another possible cause would be if the air intake was disconnected.
I'm just trying to find other things it could be other than an exhaust or catalytic converter problem. One thing you can do to see if it's the cat is to drive a short distance, then look under the car at night- if the cat is glowing red- that is very likely to be your problem. By a short distance, I mean down to the end of the block and back. After driving for a good amount of time, a normal cat will glow deep red. A bad cat will brightly glow red after a short distance.
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Moderating everything now, let me know if I can help. To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.