It depends on whether you ever intend to have it tested for emissions again. This varies widely from state to state, but in general a totally no-cats car is illegal in all 50 states unless it is older than 1978 or a historical vehicle... or it is a motorcycle. Certain classes of trucks are also exempt.

 

The modification of emmissions systems falls under Federal law which does not allow aftermarket cats unless the OEM is damaged and cannot be replaced by OEM. So either way catless or aftermarket cat you are not in compliance with the law, so you might as well go catless!

 

No, that isn't the case. If the replacement cat is CARB certified, you can be in compliance. Also, as I noted, enforcement does vary widely from state to state. Certainly you've got a better chance of getting away with an aftermarket catatlyst in Montana or Illinois vs. California or New York.

 

CARB is meeting only California Requirements which does not put you in compliance with the federal regulation. It is the federal regulation that does not allow the modification and also holds the highest fines. California only allows afermarket CAts if the original OEM emissions warranty is past it's term. For most new cars that is between 5-8 years. At that point you have to prove that your cat was failing. So my point is unless you have an older car with a documented catalytic converter failure, you must use an OEM cat.