2018 Camaro SS exhaust

I am working through the EFI tuning courses slowly but surely. I have experience as a mechanic but tuning has never been a part of this. I have a 2018 Camaro SS with the 6.2L L99 and automatic transmission. It is for the most part stock, but i recently put on a larger cat back exhaust system, with an electronic duel exhaust switch. This of course has triggered the check engine light. The code is of course a catalytic converter code. I understand how the larger exhaust affects air-fuel ratio. I have an HP tuner MPVI3, and I even watched the course on reflashing with this tool. However, I still am not sure what parameters might need to be adjusted. The VCM software gives me the option to turn off the DTC codes that are the issue, but I thought there must be a better way. I live in an area where there isn't a lot to offer for assistance for things like this. I do not have access to a dyno and plan on many more mods to come in time.

First of all you need to find out what engine check code represents exactly. Based on that information you'll be able to get a good advice on farther action and changes required in your tune...

That's actually unusual as a cat back exhaust with the stock cats retained shouldn't affect cat operation and hence shouldn't pull a DTC. As above, what codes are showing? You can disable the cat fault codes but this shouldn't be necessary.

P0420: catalytic system efficiency below threshold

I assumed the same and I do have to admit I didn’t put the system on myself but I don’t have a lift and was hoping it was just a tuning issue.

It could be low pressure pump in the fuel tank going bad...

Before anything else, I'd check, or have checked, that the wiring and connectors are undamaged and in place. I wouldn't be surprised if something was overlooked.

2018 is late model OBD and emissions so it could be tricky.

I’d have a look underneath to see what type of post cat sensor is on there. It could be a pressure based monitoring system which would mean that because the cat-back has improved flow there’s less back pressure causing it to go outsider the monitoring window. It could also be a post-cat lambda sensor system. You could do it having a look. Solution for lambda is a pretty easy fix with a wideband controller that outputs 1v narrowband signal for your ecu. If it’s a pressure sensor set up you’d have to do something like wire in a pot and wind it up into the MiL light goes out, measure the voltage and resistance then solder in a resistor to suit