Is a Sports Cat Legal in the US?

Conditional

A high-flow "sports cat" is legal in the US only if it is a compliant catalytic converter. It keeps a working cat in the system, but the cat itself has to meet EPA standards, and in California and CARB states it must be CARB-approved.

Why?

Because a sports cat retains a catalytic converter, it avoids the outright tampering problem of a decat. The catch is that not every aftermarket cat is a legal one: federal rules require replacement converters to meet EPA standards, and a very sparse "race" cat may not clean enough to qualify.

California and the CARB states are stricter still. There, an aftermarket catalytic converter generally must carry a CARB Executive Order or be a proper OEM-equivalent unit to be street-legal, and a smog check will reject a non-approved cat.

So a good-quality, EPA-compliant (and, where required, CARB-approved) high-flow cat can be road-legal, while a non-compliant "race only" cat is not. The label on the part matters as much as the fact that a cat is present.

What decides if it's legal

A catalytic converter must remain, and it must be a legal, compliant unit.
Replacement cats must meet EPA requirements (and CARB in CARB states).
In California/CARB states the cat must carry a CARB EO or be OEM-equivalent.
A missing or non-compliant cat is a federal violation.

Does it depend on your state?

The federal requirement for a compliant cat applies everywhere; California and CARB states add the EO requirement, so a sports cat that passes in a non-CARB state may not be legal in a CARB one.

Related legality guides

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Sources

This page is general guidance, not legal advice, on the rules in the US for sports cat. The detail varies by exact vehicle and changes over time - confirm with your insurer and the EPA, NHTSA and your state DMV before modifying.