How poorly will my 2015 Limited run if I replace my downpipe with a catless pipe, but I don't reprogram the PCM? Will the computer adjust on its own, or will it run horribly until I flash a new program that accounts for the lack of a catalytic converter?
The one thing with these vehicles (WRX’s and STi’s) are that the ECU is very precisely balanced from the factory (albeit very conservatively). Any change, other than a panel filter results in the need for the ECU to get flashed.
I really recommend researching and reading up on all things 2015+ WRX. You won’t go wrong learning everything you can about these vehicles. I had my plan all but erased after reading articles and videos of what can go wrong on certain mods.