GLC Hybrid Electrical Issues Undervoltage Warning

I had a similar issue with my GLC, though I can't recall the exact mileage. The undervoltage message is a pain. Check the alternator output; that's where my problem was rooted. A faulty alternator wasn't properly charging the battery, leading to all sorts of weird electrical gremlins. Have you noticed any unusual noises from the engine, especially when starting? And when you say 'checked the battery terminal,' did you just visually inspect them or actually clean them? Corrosion can be sneaky.

Thank you for the quick response. I only visually inspected the battery terminal connections, I did not clean them. I haven't noticed any unusual noises from the engine, but I will pay closer attention when starting. How did you diagnose the alternator issue? Did you use a multimeter, or did you take it to a workshop?

I initially used a multimeter to check the voltage at the battery while the engine was running. It was significantly lower than it should have been. I wasn't confident in my diagnosis, so I took it to a workshop to confirm. They ran a proper diagnostic on the charging system and confirmed it was indeed the alternator. I'd advise you do the same rather than throwing parts at it.

I appreciate the advice. I took the car to a workshop, and after their diagnostics, they confirmed that the auxiliary battery was heavily discharged and defective, as suspected. Replacing it resolved the undervoltage issue and all related symptoms. The total repair cost was 290€. Thank you again for pointing me in the right direction!