Hi Guys,
Haven't been on in a while. Sorry for being a slackazz!
AS for WB location, cats, etc. Couple things to remember here. First, Aftermarket high flow cats aren't very efficient. They don't work real well when they do work. If you wanna see when they are and are not working, log your upstream and downstream O2s. When the patterns match, the cats aren't working.
Next is, tailpipe sampling with WB O2s. Under WOT the cats "turn off" so there is no real substantial difference between upstream or downstream sampling. Sure there may be a couple tenths of a point (.2) but considering we aren't using lab grade sensors and gas benches, that's close enough for government work LOL. The reason they turn off is there is too much volume running through them ,and they just can't process it.
It's also important to understand the relationship between fuel trims and catalytic converters. Most people think fuel trims oscillate in order to keep A/F around stoich. While that is a convenient side effect, they actually oscillate to alternately douse the cats with oxygen. The main job of the cats is to store and release oxygen (oxidation), the second one being reduction, which also requires oxygen.
In the first graph, you'll see 2 oscillating wave forms. The lower one is an upstream O2, the upper one being downstream. In this scenario, the cats are not working. Notice the slight time delay from the upstream to downstream reaction.
In this second one, this is in decel mode, and fuel cut off. You can see the distinct delay from the upstream to downstream reaction.
In this graph, we have an operational cat. Notice the downstream flat line. What would appear to be a "rich" condition reported by the downstream O2 (higher than .500mv) is actually what it should be showing. Remember it's an OXYGEN sensor. It can only report what leftover O2 it sees. Also remember it's the job of the cats to store oxygen. So, in this case, the sensor is reporting an OXYGEN DEFICIENT condition, which is what it should be when the cats are working.
The main reason that aftermarket headers and cats set P0420/P0430 codes is because the cats don't work well, and the computer ends up seeing a pattern most similar to the top graph.
Graphs courtesy of edcmat-l1's former days as an emissions/diagnostics specialist LOL