Hope this helps someone: I have a 2004 C180K petrol manual, ran fine and economically for years (easily over 50mpg on a run)
Then last Sept my engine management light came on and consumption fell.
I took it to a an independent specialist, they suspected and replaced front lambda, fault (light) kept returning.
I took it to my trusted local garage more recently, they found split hoses under and above supercharger causing rich running hence higher consumption, repaired, still the same! Replaced front & rear lambda's and MAF sensor, still consumption stayed high but warning light stayed off.
Had to replace cat as running rich ruined it.
After all the causes fixed, nothing they did would bring the rich running down, still failing emissions for MOT. ECU reset on Mercedes STAR machine.
In the end, an independent MB specialist came up with the answer (MB, Medway, Rochester, Kent highly recommended!)
It seems some early C180K's were fitted with a Siemens ECU, and these have a split memory, and it was remembering that it had to richen the mixture to compensate for the air leaks.
A software update and reset solved it, back to low emissions and good mpg again now!