[ingalls]

You would notice as soon as you turned on your car. It would sound like a roaring sound from under your car and would get worse any time you give the engine more fuel. You can drive a car without a catalytic converter but it is almost universally illegal and you assuredly won't pass emissions.

[bittercynic]

>You can drive a car without...

There is at least one case where this doesn't hold:

On a 3rd generation prius the coolant loops around the cat to warm up the engine faster, and the coolant will leak out if someone steals your cat. The engine would be destroyed pretty quickly if you run it without coolant I think.

[rmetzler]

You would think there is something wrong but you wouldn't necessarily think there was a part stolen. We only found out there was something stolen when the mechanic sent us pictures from underneath the car. It was his first of these cases.

[lotsofpulp]

Emissions inspections are not required in most rural areas, and not required in quite a few state (FL/WA/MI come to mind as populous places that do not require inspections):

https://en.wikipedia.org/wiki/Vehicle_inspection_in_the_Unit...

I would just straight pipe it if someone cut out my cats. Although, my cats have been dead for years, so I wonder if they would even get paid for them.