Mercedes ML Hybrid Terminal Thirty Issue

Terminal 30 issues on hybrids are tricky. Did you fully isolate the battery during your fuse box check? A parasitic draw from the hybrid system can mimic a bad battery. Also, double-check the circuit breaker specifically for the Terminal 30 circuit. I've seen them test okay with a multimeter but fail under load. What voltage are you seeing at the battery terminals both at rest and while attempting to start?

Good point about the load test. I only checked voltage at rest. Where is the circuit breaker located for the Terminal 30? Is it integrated into the fuse box, or separate along the wiring harness? I recall chasing a similar issue on an older S-Class and it was a corroded connection hidden deep within the harness.

It's typically integrated into the main fuse box, but Mercedes layouts can vary. Given the intermittent nature and your experience with corroded connections, I'd advise a thorough inspection of the wiring harness, especially near the starter motor and around any grounding points. Intermittent issues like this are often best diagnosed with STAR diagnostics. A visit to the workshop might be worthwhile.

Thanks for the insight. You were right about it being best diagnosed at a shop. They confirmed it was a heavily discharged and defective auxiliary battery, impacting the entire system. Repair cost was 290€. Appreciate the help!