So many techs fear these group of codes because they have been bitten before by them. In my opinion, these are some of the easier codes to solve if you attack it systematically. The first step is to know the enemy. The general code set criteria is as follows; When the the downstream oxygen sensor is within 80% of the activity of the upstream oxygens sensor during a catalyst test the code is set. The key to this is graphing our scan data. Years ago scan data was much too slow to trust graphing it. But, todays datastreams and professional scantools make this a trustworthy venture. Here is a snapshot of`data that was graphed.
Another item to be wary of is exhaust leaks ahead of the catalytic converter on codes that never seem to be fixed. An exhaust leak ahead of the converter no matter small will introduce outside oxygen into the exhaust stream. Causing problems and ruin your day. A smoke machine can be used to find small leaks. So what if you have a A/F ratio sensor upstream instead of a conventional sensor. Just look at the downstream sensor graphed and use the hints I have given to diagnose these cars. Most techs think that the catalyst monitor is run during steady highway speeds. That is true of most cars. But, late model GM vehicles will run it at an idle after a highway run. Know your enemy, check your drive cycle routines to determine when it looks at converter status. I hope this helps the next time you have one of these in your bay.