Coolant loss and check engine light on Mercedes ML diesel

I had a similar coolant disappearing issue with my BMW. First, check for visible leaks around the water pump and coolant reservoir to rule out the obvious. Also, have you checked for any signs of coolant mixing with the oil? What does the temperature gauge read?

I don't see any visible leaks. The oil looks clean, no milky residue. The temperature gauge reads normal. Could it be air pockets or maybe even the head gasket, despite no other symptoms?

Air pockets are a possibility, especially if the system wasn't properly bled after a service. But given the age and your error code pointing to an electrical issue, I'd lean towards a faulty sensor connection on the cooling fan or the coolant level sensor itself. Have a workshop check it before it gets worse.

Thanks. I had it checked, and it was indeed a defective plug connection as I suspected. The repair cost was 95€. It's all good now.