My 260e failed OH emmisions . It had been sitting for a few months - so that was part of the problem. The other was the O2 sensor was bad.
I replaced the O2 sensor and drove the car up and down I-90 real fast for about an hour before taking it to be tested. The retest numbers passed - and were drastically lower than those of the failed test.
I don't know if you want to do this on your Mercedes - but when I was in college and my '82 Buick failed PA emmisions, I ran the tank almost empty and then loaded it up with that stuff used to prevent fuel line freeze-up - which I suspect is ethanol. (I got about 5 bottles of it real cheap in the spring when Big Lots was having clearance sale.) Since I was burning mostly ethanol, the CO emmisssions dropped as did HC levels. It passed. Who knows what it did to the engine - and who cares? After all, it was a 1982 Buick Skylark.