Was on my way into work this evening (I work nights). Stopped at some traffic lights & as I pulled away again there was a loud rattling from under the vehicle for a few seconds that suddenly stopped. At first i thought part of the exhaust had dropped off. It was then that I noticed a strong smell, like rotten eggs that gradually disappeared over the next mile or so. When it was safe to pull over I stopped & had a look underneath but could not find anything obviously loose. Only thing I could think of is maybe the catalytic convertor is breaking up inside & perhaps a piece of it came loose inside & lodged in the muffler? The smell I got was certainly like the smell that catalysts produce sometimes - especially when they're new. The vehicle still performs normally & there is no "Service Engine Soon" light on to suggest there's a problem.
Any suggestions would be welcome.

 

That would be your catalytic converter failing. Prepare to spend about $400 to replace it.

 

Thanks for the reply.
I had a feeling that it might be the convertor on it's way out. I am tempted to wait until the annual emmisions inspection in October to see if it passes the test or not. If it fails I would have to replace it anyway. After what happend earlier this evening I reckon it's on borrowed time. Like I said, the "Check engine" light is not on at the moment - would have though that it would come on as soon as the post-cat oxygen sensor noticed a problem. I have 3 oxygen sensors on mine, one on each downpipe & one after the catalyst. I don't know if this is just the case on European models or whether the North American models are the same? I am assuming that the one past the convertor monitors converter performance & turns on the light if something is seriously wrong?

 

If you are close to Chicago I know a place that can get you one AND put it on for $100!!!

 

Thanks for the advice but I am across the atlantic in southwest England, so a little far from Chicago. There are several companies here in the UK that sell "universal" stainless catalytic converters at less than half the price of a genuine GM one. It involves having to weld the correct fittings on each end but that would be no problem as I am an engineer in a food factory so have access to the necessary stainless welding equipment. When I do get round to replacing it I may change the muffler as well if lots of the converter internals have gone missing. It's the original muffler so it's over 10 years old now, probably time it got replaced. Been thinking about the problem I had earlier & it might help to explain a rattle that I have had for months at low revs when pulling. have always blamed the PCV valve but i'm not so sure now. Wonder if it's been the converter all along? I'm finishing my nightshift in 2 hours time so will be interesting to see if I get problems driving home again?