Bad Catalytic Converter?

You can't it's always going to pick up on the lack of the cat and it's breaking the law to run it without it.

I see your point there and frustration with me they just take my whole car and total it. But there isn't a way to bypass the cat that the computer won't know. Are they just unbolting the cat's and taking them that way?If that's the case I would have the nut's and bolt's welded in place that's a option. Doesn't stop them from cutting it off with a pipe cutter though.

A universal is iffy.

You might have air injection on the convertor or a o2 sensor on it, and if you dont have the proper one you n eed to do modding.

Plus it wont fly if you need emissions testing.