BMW M4 Dead ECU: Terminal 15 Ignition Issue Help

Hey there! I have some experience working on BMWs and had a similar issue with my 2018 BMW M2. The symptoms you're describing sound very familiar. I took my car to a workshop when it showed similar starting problems and error codes. Turned out the cable connection to the ECU was damaged, causing intermittent power issues. The fix wasn't too complicated, they replaced the faulty wiring and connectors, which cost me around 95 Euro. Before I can give more specific advice, could you share: Are there any other electrical issues you've noticed? What error codes are you getting? Did this start happening suddenly or gradually? By the way, if you want to avoid workshop visits, I'd recommend against DIY for ECU-related problems on these cars, the electrical systems are quite complex and sensitive. Let me know those details, and I might be able to help further.

Thanks for the detailed response! Just had my latest service done last week and sitting at 143125 KM, so this issue caught me off guard. No other electrical problems before this, it happened suddenly yesterday morning. Getting error codes P0688 and P0687 through my scanner. The battery tests fine, but I'm really hoping it's just a wiring issue like in your case. These ECU problems can get pretty expensive on M cars, so fingers crossed it's something simple. Definitely planning to have a pro look at it rather than messing with it myself.

Hey again! Thanks for sharing those details. Those error codes (P0688 and P0687) are definitely related to ECU power supply circuits, just like what I experienced with my M2. Since you mentioned having recent service work, my immediate thought is to check if anything got disturbed during that service. When my issue happened, it turned out that some wiring had been accidentally pinched during a routine maintenance job. The fact that your battery tests fine is actually good news, matches my experience exactly. In my case, the mechanic found that the wiring harness near the ECU had some wear spots that were causing intermittent connections. Given your mileage (mine was around 130,000 km when it happened), this kind of wear isn't unusual on these models. I'd strongly recommend getting it checked at a workshop that specializes in BMW electronics, even better if they have experience with M cars. The diagnostic process for my M2 was pretty straightforward once they knew where to look, and the fix was relatively quick. Just make sure they thoroughly check all the connections and wiring routes to the ECU, as these can be tricky to spot. Keep us posted on what they find. These issues can be nerve-wracking, but they're usually fixable without breaking the bank too badly!

Thanks so much for sharing your experience, it's really reassuring to hear your similar situation had a straightforward fix! I've just booked it in with a BMW specialist for tomorrow morning. Hopefully they'll find the same kind of wiring issue you had. You're absolutely right about the recent service possibly being connected, I'll definitely mention that to the mechanic. The wear and tear at this mileage makes sense too, and I'm feeling much better knowing it might not be a complete ECU failure. Thanks again for taking the time to help out with such detailed info. I'll update the thread once I get it sorted, so others with similar issues can benefit from the solution. It's great to have helpful like you in the community who understand these tricky M car problems!