I know this thread is played out but I'll contribute my 2 cents.
My check engine light has plagued me constantly for the past year. There is nothing more annoying then driving a Benz around with a check engine light that is constantly on.
Any who... I always tightened the cap, and followed the directions so not to upset the onboard computer god. Twist it, and make it click 3 times. But the light still illuminated. Annoyed.
Until one day I saw the light .... when a dainty, demur female gas attendant filled up my tank... I mean my ml's tank... and really twisted the cap hard 3 times. I was just making the ratchet click three times... when in reality you have to really twist the cap hard... 3 TIMES. I'll blame it on the technical writing in the manual, but the light has not been on since.
I guess now's a great time to sell it!