It depends on your state and the converter's certification. Federal and many state rules require converters that meet specific emissions standards, and California (CARB) rules are strictest — a cat legal in one state may be illegal in another. Look for the correct EPA or CARB certification for your area, and never run a straight pipe or gutted cat on a street vehicle. Verify local law before buying.

A little, mostly on modified engines with other exhaust upgrades. A high-flow catalytic converter reduces exhaust restriction, but on a stock truck the gain is small — the factory cat usually isn't the main bottleneck. The bigger benefits come when a cat is paired with headers and a freer-flowing exhaust. If you're chasing power, treat the cat as one piece of a complete exhaust plan, not a standalone fix.

Watch for a check-engine code, failed emissions test, rotten-egg smell, or sluggish performance. A clogged or failing converter restricts exhaust and trips catalyst-efficiency codes, while a rattle inside often means the internal substrate has broken up. Confirm with a diagnostic scan before replacing, since sensors and upstream issues can mimic cat failure. When it does need replacing, match a converter certified for your state.