You will need to start by reading or having read, the self-diagnostic fault codes in the ME engine management system.
These fault codes will give you a basis for diagnosis. Cheap OBDII scan tools are available for laptops or palm readers. Partsshop has one for the palm devices. I would highly recommend one of these devices for anyone interested in viewing their system on a 1996 or newer car (C class from 94). For the engine systems they give as good of info as professional tools in the mega thousands of dollars. Unfortunately this is because of government rules which do not include body or chassis or climate systems.
Bosch Master, ASE Master, L1
33 years MB technician