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GenSet DOC or Muffler on Old Unit

Got an old 3412 GenSet with 6FA05914 enginer serial number and 545 kW. Tested unit for RICE and got >70% CO reduction 3 years ago (300 ppm from engine to about 50 ppm out exhaust). Retested 3 years later (recently) and getting zero reduction. Young CAT mechanic says problem is that the DOC is NOT a DOC but a muffler. I can't find any manuals on this thing and was wondering if anyone could tell me if this is a muffler or DOC and if it is a muffler, how in the heck did we get a >70% reduction 3 years ago. Many thanks. 13.JPG14.JPGNameplate3.JPGNameplate4.JPG