Ok so the cats died in my bf
What I need to know is if I weld in a higher flowing cat (not a gutted cat) on the standard exhaust section. Will it throw up the engine emissions light. Or is it only when the second o2 sensor is unplugged on aftermarket exhausts where the second hole doesnt exist
Thanks

it *may* still log a fault code at times.
The cars run a self test proceedure while driving which changes the fuel mixture slightly and looks for a change in readings of front and rear oxygen sensors at a rate which corresponds to the standard cat.
With a high-flow cat it will see the rear sensor change too quickly and will log a P0420 (Catalyst efficency bank 1).

If you want to know just how strict the emissions system is google that code number (Or P0430 which is bank 2 on a V config engine)