BMW 5 Hybrid Terminal 15 Starting Issue, ECU Help Needed

Hey there! I have some experience working on cars, and went through something really similar with my 2014 BMW 3 Series Hybrid last year (also owned a 4 Series previously). The Terminal 15 issue sounds exactly like what I faced. Mine showed the same symptoms, wouldn't start and had ECU voltage errors. Turned out it was just a loose connection in the wiring harness that connects to the ECU. My mechanic found the faulty connection, replaced the damaged cables, and properly secured everything. Cost me around 95 Euro and took about 2 hours to fix. Before I can offer more specific advice, could you share: Does your battery charge indicator show normal readings? Are you getting any other warning lights besides the ECU error? Has the car had any recent electrical work done? These details would help me understand if you're dealing with the same issue I had.

Thanks for the detailed reply! I had my last service about 3 months ago at 40522 KM, and everything was working fine until now. To answer your questions: battery indicator shows normal, no other warning lights except the ECU error, and no recent electrical work done. Based on your experience, it does sound similar to what I'm dealing with. The loose connection theory makes sense since the issue came up suddenly. I'm relieved to hear it might not be something major. I'll probably get it checked out soon to avoid any long-term damage, but it's good to know what to mention to the mechanic. Your insight about the wiring harness is really helpful, I'll definitely have them look at that first.

Hey again! Thanks for those extra details about your BMW. Yeah, this is sounding more and more like what I dealt with last year. The fact that you're seeing the same isolated ECU error with normal battery readings, just like I did, really points to that wiring harness issue I mentioned. I remember when mine started acting up, it was also pretty sudden, working fine one day, wouldn't start the next. The timing of yours is interesting though, three months after service. Makes me wonder if something might have gotten knocked loose during the maintenance work. Here's a tip from my experience: when you take it to the shop, ask them to specifically check the connection points where the wiring harness meets the ECU. In my case, there was some minor corrosion at one of the connection points that wasn't immediately visible. They had to clean that up along with replacing the damaged section of the harness. Also, good call on getting it checked soon. I tried driving mine for a few days after the first signs, and it just made the connection more intermittent. Definitely not worth risking any additional damage to the electrical system. Let me know how it goes when you get it checked out. I'm curious if it ends up being the same issue as mine!

Thanks so much for all this information! You've been super helpful. I just booked an appointment with my mechanic for tomorrow morning and mentioned the wiring harness connection issue specifically. You're right, maybe something did get knocked loose during that service three months ago. I hadn't thought about that possibility. I'm definitely not going to risk driving it anymore until it's fixed. I've already called for a tow to get it to the shop. Really appreciate the tip about checking for corrosion at the connection points too, I'll make sure to have them take a close look at that. The cost you mentioned seems pretty reasonable, especially compared to what I was worried it might be. I was imagining having to replace the entire ECU! I'll update you once I get it back from the shop. Thanks again for sharing your experience, it's made me feel a lot more confident about dealing with this issue.