[mikefocke]

A CE is a generic fault. Ask or find out what the specific codes are. And 30kk with "aftermarket" cats and some old O2 sensors could result in cat destruction. Dump too rich stuff into a cat where the system isn't able to adjust the mixture and 30k miles is enough to cause all sorts of problems downstream of the engine.

Look at the exhaust pipe. How black at the tips?

You diagnose by looking at a o'scope and seeing how the voltages vary. Are the front ones varying within spec? Are the rear ones low and flat. How old is the MAF? Cleaned it with MAF cleaner recently? Are the sensors Bosch or Porsche parts (Porsche just labels them, Bosch makes em)?

I replaced O2 sensors in a pair so the freshness of the measurement was equal. Something any muffler shop can do with the right parts in 15 minutes a piece.