To reset the engine light in a Merc C240:
- Remove the nut from the negative battery terminal and take the cable off from the terminal. It should not be touching the terminal so if you can't manage, just place a plastic or paper between the terminal and the cable.
- You need to empty the energy stored in the electrical system so after taking the cable off, press the horn which will help the same cause.
- Wait for approx. 10-15 minutes before connecting the negative battery terminal again.
your engine light should be reset.