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I have a 2005 f150 5.4 how can ya tell if the cats are stopped up without having to cut them off and see?
A little Vacuum gauge tapped into your Vac harness is one way to verify restriction. If a converter is stopped up, you would know it, - when it gets bad, they glow in the dark. You'll generally get efficiency DTC's before they get that bad.
A dealership is the easiest way to measure restriction/flow.