97 F-150 4

Failure of one catalytic converter isn't real common so there's no need to replace one that hasn't failed. It's important to point out that fault codes never say to replace parts, but this may be the closest thing to an exception. They only indicate the circuit or system that needs further diagnosis. Starting with '96 models, the Engine Computer monitors the efficiency of the converters. Usually when it sets the "efficiency" code, other conditions that can mimic the problem have already been ruled out. A too-rich or too-lean condition would be detected first and would block the "efficiency" code from setting. Said differently, it's likely there is no other underlying problem, and a new converter is all that's needed.