Mercedes E Klasse Diesel Coolant Issue Resolved

Have you checked the coolant level sensor itself? Sometimes those can fail and give false readings. Is the coolant actually low, or is it just the sensor reporting it as low? Also, have you noticed any unusual smells, like exhaust, coming from the coolant overflow tank? Is the engine overheating?

The coolant level is indeed low; I've had to top it off twice in the last month. I have not performed a smell test on the coolant overflow tank. Where is the coolant level sensor located, and is it easily accessible for inspection or replacement? What am I looking for in a smell test?

The coolant level sensor location varies, but it's usually on the coolant reservoir or radiator. A faulty sensor is possible, but the fact you're losing coolant points to a leak. The smell test is to detect exhaust gases in the coolant, indicating a possible head gasket leak. However, given you're losing coolant and have a diagnostic code, I'd advise a workshop visit. They can pressure test the system and pinpoint the source. It is probably difficult to find with the naked eye.

Thank you for the advice! I took it to a workshop, and they identified a small leak at the water pump. They replaced the pump and related seals. The final repair cost was 95€. No more warning lights or coolant loss. I appreciate your help!