I would at this point forget about the spark plugs, and return the injector cleaner - I have 107k miles on my C230 and never needed to use or clean my fuel injectors - though I make an effort to use quality gas from a busy station.

I have however needed a MAS semsor at around 70K-80K miles or so.


Also www.club202.com is a good resource, though not as good as this site