I just replaced the cats on my 00' E320 and can tell you there is a difference in quality from aftermarket suppliers. The right side I purchased is DEC brand. It does the job but really wasn't the quality I was hoping for. The left side I bought is a CAT brand and the quality of that thing is EXCELLENT. All stainless and looks to be of high quality. Oh and the best part is it was a little cheaper then the DEC brand that I had bought. I bought it from Import Catalytic Converter Warehouse.

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86 Nissan 720 P/U - 285K miles and counting!!
91 Volvo 240 - 218K miles and still going strong.
00 Mercedes E320 - 111K miles and lovin it!!!