I recently had a P0430 come up on my 2000 S-Type. I had this take place one other time about 2 months ago and both times the fault cleared. I am assuming a loose connection and the question is where to start looking?

 

Doesn't have to be that (but just about could be). What are fuel trims like, and all O2 readings (preferably graphs / waveforms)? A bad O2 sensor could mislead the PCM. Bad would include wiring, of course.

 

Gus,

If you have or can get hold of a scantool that can graph, here is what to look for to confirm the cats as good or bad. This would hold for any car that uses the "universal type" oxygen sensors.

Drive the car and get it to full operating temp, then let it idle for several minutes to settle the fuel system, then look at the O2 sensors.

The first screen shows the downstream O2 signals looking more or less like the upstream, though bank two is not as bad as bank one. The second screen is after replacing both cats. If the cats can store oxygen like they are supposed to, the downstreams are mostly flatlined.

Cheers,