Mercedes M Klasse Coolant Issue

Low coolant is never good. It could be a simple coolant reservoir leak, those plastic ones can crack. Or the radiator cap isn't sealing properly. If you're unlucky, it could be the head gasket. Have you seen white smoke from the exhaust? Is the engine overheating?

I haven't seen any white smoke, and the engine temperature is normal. But I'm still losing coolant. Is there a good way to find the leak? Should I worry about the error memory saying there's a bad cable or plug?

No smoke or overheating is good, but I wouldn't rule out the head gasket completely. Finding the leak can be hard. They can be small and evaporate fast. You could try a pressure test. Given the error code about the cable or plug, and that you added coolant, I'd take it to a shop. They can diagnose it right, especially if you don't know cooling systems well.

Following your advice, I took the car to a mechanic. The cable connection was faulty, just like the error memory said. The repair cost 95€ Thanks again!