The engine light came on on my mercedes. My car is the C180 Kompressor (S203 W203 M271)
A quick diagnosis showed trouble code P0135. In my case, it is the upper oxygen sensor that has failed. The oxygen sensors are easy to change, but require patience as the space is tight and the sensors themselves can be incredibly difficult to detach from the exhaust. I have not filmed the whole process but show part numbers, where the censors are and where the cables and connectors are. All the best and hope this can be of use to someone.

The error disappeared after the oxygen sensors were replaced and the fuel economy improved slightly. So it is not worth waiting to replace these censors.

00:00 Intro
00:20 view of oxygen censors
00:37 part number upstream censor
00:48 part number downstream censor
00:58 under car view downstream censor
01:51 under car view upstream censor