I used "de-carbonizer" last year on my W124 to "aid" in passing emissions.
Granted, a clogged EGR tube was the real culprit, and after I had that taken care of, I never really had a chance to drive the "snot" out of the car prior to the retest. So I put the stuff in the tank and drove the car as much as I could to get the cat warmed up...the car squeaked by the emissions test!
That kinda disappointed me, as it always passed with room to spare in previous years...even when I had the leaky head gasket and other issues...