too much fuel producing soot probably plugged the CAT.
you have two choices, drill out the matrix in the cat, if you want to do that it requires a long drill bit on an extension, preferably a half inch drill motor and a long nosed pliers to twist off the mesh as pieces of the ceramic material break off, or a new CAT.
Bought a car that had been driven frequently in short distances, not allowing the CAT to heat up. Tried a Italian tune up but still sluggish.
I dropped the exhaust line and tied putting a propane torch to the plugged CAT thinking that might burn off the crud but it didn't work, a futile effort on my part. That's when I learned how difficult it was to remove the matrix. Surprisingly little increase in exhaust noise though.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!