When to replace cats?

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What are cats? :shrug:
In all seriousness, when your car starts getting really down on power, and you can trace it back to a restriction in your exhaust, it's time for them to go. Once you've determined that all systems check out ok, you may want to look at the cats as a suspect if your car starts bogging or dying when backpressure in your exhaust gets too high and causes the engine to stall. Your gas mileage will definitely take a hit as well, since the engine is working harder to try to force exhaust past the restriction.