I learned (almost the hard way) that most catalyctic converters are replaced unnecessarely. The one on my 560sl is worth about $3,500 Cdn to replace.
What I would strongly suggest (it worked for me when mine failed) is spend about five bucks for a General Motors product called "Cleans" or "Cleens" Pour it (one or two cans) down the throat of your air intake when the engine is running at a fast idle. Pour the last bit down quickly enough to stall the engine. You must let the car sit overnight now for best results. When you start it the next day the neighbors may think your house is on fir because there will be lots of smoke. Then take it for a good run on the freeway.
I've seen at least half a dozen success stories from this treatment and I know several good garages in my area that do it rather that hit the customer with a huge bill for catalictic converter repalcement.
Hope this helps