All vehicles registered in, or commuting at least 60 days into, Bernalillo County are required to pass an emissions test every two years. Part of the emissions test is a visual inspection to confirm that the vehicle has a catalytic converter(s) installed.
Outside of Bernalillo County, state or local laws do not (as far as I know) require catalytic converters, but Federal law may prohibit you from removing your cats from your vehicle.
Federal law requires all cars as built after 1975 to be equipped with a catalytic converter.
catalytic converter
All cars made since 1975 in America have a catalytic converter.
The main usage of a MagnaFlow Catalytic Converter or any other catalytic converter is commonly the reduction of toxic emissions in the exhaust systems in cars.
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Absolutely not. The fumes released from any engine can be deadly without a catalytic converter and severely harm the environment no matter how efficient the engine is and no car can legally not use a catalytic converter.
If your question is "Does a diesel car have a catalytic converter" then I might be able to help. Some countries require catalytics on diesels, and some US states also require them. Most newer cars and trucks will have them mounted. A diesel engine will actually run better without a catalytic, but having one will cut down on the black soot coming out of the exhaust pipe. California I know for sure will start to require smog checks on diesels. one way to automatically fail is if you have removed your catalytic converter. I've owned several diesel trucks...
Catalytic converter
Almost all cars have only one catalytic converter. Some can have two, four, etc.
Should you cut off the catalytic converter? No. But can you and will SOME vehicles actually benefit? Yes. It is illegal to remove any emmision control device from your vehicle in most if not all states. For countries such as Mexico where it is not illegal, some vehicles without downstream Oxygen sensors (after the catalytic converter) it can have some benefits. It is really the same benefit as a free flowing exhaust. Modern (97+) cars already have a free flowing design catalytic converter so there is no benefit to doing this.
China adopted Euro 4 emissions standards in 2010, Europe in 2005, so they aren't that far behind. You cannot come close to Euro 4 without a catalytic converter. The max CO level for a Euro 4 car is about 0.5 I think and from memory a car without a catalytic converter (pre 1992 in the UK) is allowed upto 3.0 CO.
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to limit the air pollution the car puts into the atmosphere.