I Have a 1998 bmw 323i convertible, right now i have the stock exhaust on the car but i bought an aftermrket muffler to be welded onto the car the mechanic said if he cuts my muffler off to weld the other one on my check engine light will be on and i will have problems with some valve being open, how much of this it true and what exactly is he talking about

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Can you get a description of this valve? It's possible he means the catalytic converter, which if removed would turn on the check engine light.

- Nick