Recently my '09 traverse threw a CEL. I checked the code with my pocket reader and it shows: PO430: Catalyst system efficiency is below threshold. Band 2 The car runs fine. So in an effort to make a quick fix for emissions testing reasons, I put a 02 spacer on the downstream O2 sensor on bank 2. Which usually works, but the code came back. What I find strange about this vehicle (or maybe it's more common than I know), there is a cat directly after each exhaust manifold. Then there is an O2 sensor. But there is also another cat after these two sensors. Shouldn't the downstream 02 sensor be after that? I mean, if they're really trying to figure out if the emission coming out the tailpipe are at an appropriate level.... My reader only checks codes. Bad cat? Try changing O2 sensor? I also took it to an O'Reilys, to see if they could find out more. They just said either O2 sensor or it needs a new cat.