I have a 2001, 4.3L, with 165,000 miles on it. I get good mpg (19.6) on my 50 mile round trip to work. I am pretty sure I have a bad catalytic converter. I used to get the sulfur smell, and I have the rattle underneath in the area of the CC. My only problem is I get a stumble at idle after she warms up while I am stopped at a light for 30+ seconds. I get no check engine light issues. My question is if I just ignore the stumble at idle, can I keep running it this way or do I have to spend the $ to replace the CC before it causes other problems like stranding me on the side of the road?

 

Having a bad Cat the car will probably not run as efficiently. Also, if you live in a state with emission laws you will probably fail the inspection with a bad cat.

 

If you can get under the vehicle with a rubber mallet smack the converter and see if you can duplicate the rattle. If it sounds like its coming from in the converter then its toast. The innards are breaking apart and it should be fixed. I'm surprised you aren't throwing a p0420 or p0430 (catalytic inefficiency) yet if this is whats going on.

 

I got the same car, and same problem. How much does it generally cost to get it replaced at a shop?

 

A rattling cat can go a long LONG time before it causes a DTC. Don't ask how I know this.

I'd guess, and mind you it's only a guess, that it's somewhere near $500.