[cmiovino]

There are two O2 sensors. The first is "pre-cat" and is really "air-fuel ratio" sensor, with the main job of monitoring the exhaust and adjusting fueling. It doesn't throw the P0420 code - but it will trigger other codes of your mixture is too rich/lean, etc.


The post-cat sensor (aka "downstream") is in the front pipe behind the secondary cat there. It's purpose is to strictly look to see if the cat is doing what it's supposed to be doing and a main job emissions (aka throwing the P0420 when it's out of tolerance). It does not do anything with fueling.


If installing a new OEM header fixed the P0420, the cat in the header might have been bad, but I don't see how if it was only 10k miles old. Either way, the rear O2 sensor didn't like what the cat was doing.


I don't know a ton about voltages, so I'll refrain from speaking on that. As far as fuel trims however, there are long and short term fuel trims. Short ones modify the long term values in real time. Long term fuel trims are learned values that allow the car to run in different fuel (by state, octane level [91 vs 93], climates, and even with different mods to some extent. No car is exactly the same. "2" is usually a percentage and ideally should be as close to zero as possible. 2% is totally fine. You just don't want something like 10%+ and at some point it will hit a tolerance threshold and throw either a too rich or too lean CEL code.


Looking at long term fuel trims can be helpful in finding issues like vacuum leaks and such, but if you're at 2% and it's all running good with no CELs, you're good to go. If you have a stock car, you can't even see these ECU values.