The Mercedes-Benz C240, like most modern vehicles, has a computer system that monitors the engine for potential problems and failing parts. When a problem is detected, it alerts the driver using an engine light on the dash. Once the needed repairs are complete, the light often remains on until it is reset manually.
Remove the nut from the negative battery terminal using a wrench or socket set. Slide the cable off of the terminal and set it aside.
Press the horn for several seconds. This depletes any surplus energy stored in the electrical system.
Wait 10 minutes before reinstalling the battery cable.
If the cable still springs back into contact with the terminal, use a nonconductive material such as plastic or paper as a barrier to keep them from touching.