Among other reasons, there is great likelyhood that the replacement cat wasn't a genuine MB part. The MB catalyst is about 10 times better made than any aftermarket version. Based on that alone two years isn't bad.

But as others have said a cat can be damaged in 50 miles of meltdown conditions. Those are such when closed loop fuel control is not strictly maintained.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician