Last summer I had the same codes appear... catalytic converter inefficiency. It was still under warranty so the dealer replaced both manifolds. The new precats work fine. I just recently had the light come on again and I think it is going to be the same problem. I have to read the codes, but if it is the catalytic converter inefficiency then I may have found a pattern.
When the first codes came on, I had been running Nos for a month or so. I had already removed the Nos from the car and about a month later the light came on. I believe it was because I ran rich quite a few times (hit the nos without turning on the bottle ;0).
This last time I ran a ton of top end engine cleaner through my engine and let is set awhile. I let it clean the pistons and intake, then I went to start the car and it wouldn't start. It cranked like it had no compression. I injected a squirt of oil into each cylinder because I suspected that the cleaner had cleaned all the oil off the cylinder walls while I let it set. The motor did indeed start right away after that, but it puffed a ton of white smoke for about 10 minutes.
A month later the CEL light came on.
It appears that the precat efficiencies can be screwed up quickly by oil or excessively rich mixtures.
Take this story for what its worth.
warmonger
