Allterrxin

Did you ever figure out your issue? I had the same thing happened to me. Replaced both cats and the check engine light came back on. Have you tried to graph the codes O2 data and see what it tells you?

Thank you! You only need one right? Since the relay harness connects to one of the original bulb pigtails only, from what I gathered.

Did you have to get error cancellers? I have a 2014 SLT AT and I'm wondering if I will need error cancellers if I go with this set up. Thanks!

I would say that too. If you haven't changed them in a while, you could do that. You can also get one or two out from each bank and check the condition and gap. I would still recommend to check the graphs first. Just looking at the pictures that were posted, it looks like the bank 2 is sticking on the high side a bit on the upstream sensor.

Were you able to resolve your issue @hysinthius? I had the same thing on my 2014 Sierra AT. I replaced cats and all o2 sensors, still had a P0430. I finally fixed, it took me about 3 weeks of just trying to figure it out and I learned a lot about cats and sensors in the process lol. One thing to do is to check the graphs on the scanner while you're driving. I bought a WIFI/Bluetooth OBD2 scanner adapter and downloaded an app on my phone that allowed me to record data and graph the data from the O2 sensors and other data. Just because the graph looks good when idling doesn't mean everything is fine. Take a couple of trips and record the data or have someone drive while you look at the graphs. In my situation, the downstream sensor was dropping more than it should and that's what triggered the code. From my understanding, the code gets triggered because of the number of the fluctuations, not the actual voltage. Downstream O2's are supposed to be more steady with some variation under load. It turns out I had a small miss because the sparkplugs were not good (I had ordered AC Delco spark plugs from eBay at a good price early this year and had someone else put them in, turns out there are counterfeit sparkplugs out there, but that's a story for another time). After replacing the spark plugs and wires with original AC Delco ones, everything has been back to normal. So it turns out I had a small misfire, not enough for the PCM to detect it, but I learned that you can detect these misfires by looking at O2 and Fuel Trim data on your scanner. On my graphs, the downstream O2 sensor voltage would drop low during acceleration, contrary to what I understood, a misfire like that would make the downstream O2 sensor voltage drop because of the additional unburnt oxygen. I thought it would go up because of the unburnt fuel, but not always. Also, reset the check engine light and when it comes back up, check the freeze frame data. You should be able to see exactly what triggered the trouble code. Another good indication of this condition for me was poor fuel economy. Check the fuel trim and O2 data, for irregularities. Check to see if your Short Term Fuel Trims spike up past 10% (lean) or so, and see if your downstream O2 drops at the same time. Check that your air filter hose/duct after the MAF is tight and properly attached. If you have more air coming in than what your MAF is telling your PCM, that would cause this issue. This was an easy way for me to check without throwing money and parts into it and messing something else up. I spent 16 bucks on the scanner and 6.99 on the app. I looked for info left and right and couldn't find anything other than people saying that a P0430 and P420 just means you need to replace a cat, but I found out that is not always the case. Besides, if you replaced your cat, that means something made the cat go bad, so you will still need to fix it or replace the act again sooner than you should. I hope this helps someone, I spent 3 weeks looking at graphs, and sensors. I am by no means a mechanic, and if anyone else knows better than me, feel free to correct me.