BMW M3 ECU Terminal 15 Voltage Issue, No Start Fix

Hey there! I actually dealt with something similar with my 2015 BMW M4 last year. I have some experience working on cars, mostly basic maintenance stuff. My car showed the same symptoms, wouldn't start and had ECU voltage issues. Took it to my regular workshop and turns out it was just a loose connection in the ignition wiring harness. The mechanic found a damaged plug connection that was causing intermittent power supply issues. Total repair cost was around 95 Euro to replace the connector and secure the wiring properly. What's your car's mileage and have you noticed any other electrical issues before this started? Also, does the car make any clicking sounds when you try to start it? This info would help figure out if you're dealing with the same problem I had.

Thanks for sharing your experience! I've got 53285 KM on mine and just had the service done last month. No clicking sounds at all when trying to start, just total silence. The dash lights come on normally though. Haven't noticed any other electrical gremlins before this started happening yesterday. Good to know it might just be a wiring issue, definitely sounds less scary than a full ECU replacement! Did your mechanic have to do any special diagnostics to pinpoint that loose connection?

Hey again! Yes, my mechanic did run a full diagnostic scan first to confirm the voltage interruption pattern. The interesting thing was that in my case, the symptoms were exactly like yours, complete silence when turning the key, but dash lights working fine. Since you mentioned your recent service, this triggers something, my issue actually started about two weeks after a routine service too. What happened was that during the service, they had to access some components near the ignition wiring harness, and the connector wasn't properly secured when everything was put back together. Makes me wonder if something similar might have happened with yours. My mechanic used a voltage meter to trace the power supply path from the ignition switch to the ECU. That's how he spotted the loose connection, the voltage reading would fluctuate when he wiggled the harness near the firewall. Pretty straightforward diagnostic process really. Given how similar your symptoms are to what I experienced, I'd bet money it's the same issue. Definitely have them check those connections first before looking at more expensive fixes. And make sure they check the entire harness route, sometimes the problem spot isn't where you'd expect it to be. Let me know what they find, curious to see if it's the same gremlin I dealt with!

Thanks so much for the detailed explanation! That's super helpful, especially the part about it happening after your service too. That's too much of a coincidence to ignore. I've just called my workshop and mentioned what you shared about the voltage meter diagnosis and the connector issue. They seemed pretty interested in checking that first. I'll make sure they do a thorough check of the whole harness route like you suggested. Really hoping it's just a loose connection, would be such a relief compared to ECU problems! The symptoms matching exactly with yours definitely makes me feel better about this whole situation. Will definitely update you once they've had a look at it. Thanks again for taking the time to share all this info, really appreciate having someone who's been through the same thing! At least now I can go to the workshop armed with some knowledge about what to check first.