Replacing the Cat makes sense. I had a truck that failed. I replaced the cat at the suggestion of an experienced mechanic and it passed with flying colors. Afterwards, I realized that the EGR was completely disabled, but it passed OK anyway.

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83 W123 240D
76 300D, "Lil Yellow Car"
1978 Cessna 172N 180 HP Mod

"If life ain't fun, I'm not interested"

The important things are those that you learn after you know it all.
Chip Foose

Build a better mousetrap and you will REALLY infuriate corporate america.