My '02 LS1 SS has been throwing the P0420 {Catalyst System Efficiency Below Threshold (Bank 1)} code recently. Today however, it also threw P0430, the same thing on the bank 2 side. I figure there's way too slim of a chance for both cats to be going out separetly at the same time. So, what could be causing this problem on both sides of the car?

 

Any chance you've done something that may have contaminated the cats - e.g. using leaded fuel, using silicone based (non sensor-safe) sealants, etc.?

 

I haven't done anything to contaminate them, but I just bought the car May 17 through a used car lot and really have no way of knowing how it was treated by the previous owner. It does look like they may have installed a catback exhaust system though.
The car started throwing the first code about a week and a half ago and today was the first time that I've seen the second code. As far as gas, I've been filling up with Exxon's 89. Should I try switching to 93?

 

Your Owner's Manual clearly indicates that although low octane regular fuel will not hurt your engine, it may reduce performance. Unless the altitude where you live is much higher than sea level, its very likely that in hot weather, you are getting ignition timing knock retard when using the low octane fuel. The engine will make more power with the 91 octane fuel recommended in the Manual.

However, that wouldn't cause a problem with the cats. Are you sure the cats are not hollowed out, with O2 "sims" in place of the after-cat O2 sensors, to elminate the codes for lack of cat performance?