Mercedes CLA Diesel Battery Drain Issue

A severely discharged battery on a modern Mercedes with those symptoms does point towards parasitic battery drain. The error codes are a key indicator. Before condemning the battery sensor, have you load tested the battery itself? A weak battery can exhibit those symptoms and throw erroneous codes. Check the battery charging voltage when the engine is running. It should be around 14V. What voltage are you seeing?

Thanks for the quick reply! I haven't done a proper load test. I assumed a nearly new battery wouldn't be the culprit. I will check the battery charging voltage as you suggested. Where should I measure that voltage to be certain?

Measure the charging voltage directly at the battery terminals with the engine running at idle. If the charging voltage is significantly below 14V, the alternator or voltage regulator might be at fault, preventing adequate battery charging. Considering the error codes and the parasitic battery drain, the battery sensor is still a strong suspect. The sensor provides data to the ECU regarding the battery's state of charge. If it's providing incorrect data, the ECU might not manage the charging process correctly, ultimately leading to a discharged battery. I would advise taking it to a qualified workshop for proper diagnosis and repair.

Right, I appreciate the advice. I checked the voltage and it was fluctuating wildly. I took it to my local shop, and it was indeed the battery sensor. They replaced it, and the total repair cost was 315€. No more error messages or weak starts! Thanks again for pointing me in the right direction.