Failing M3 ECU Symptoms, Need Urgent Repair Advice

Hey there! I feel your pain. Had similar issues with my 2008 BMW M5 last summer. Those symptoms you're describing are textbook main relay problems, I went through exactly the same thing. The starting issues and unusual shutdowns were really concerning. When I finally took it to the garage, turned out the main relay was failing, causing those electrical gremlins. It wasn't sending proper power to the ECU and other systems. The mechanic showed me the corroded contacts on the old relay. Getting it fixed was pretty straightforward once they identified the problem. They replaced the main relay and reprogrammed the ECU. Not a super expensive fix compared to what I initially feared, and definitely better than replacing the whole ECU. Before you go spending big money on a new ECU, get the relay checked first. The repair took about half a day at the shop. This isn't something you want to ignore though, could leave you stranded if the relay fails completely.

Thanks a lot for sharing your experience! This definitely sounds similar to what I'm dealing with. Good to know it might not be as bad as a full ECU replacement. Just curious, do you remember roughly how much you ended up paying for the relay replacement and reprogramming? Also, has everything been running smoothly since the fix?

Hey again! Just wanted to update you, you'll be glad to hear it actually turned out to be exactly what I suspected. Got it all sorted at the mechanic and haven't had a single issue since then. The repair was way more affordable than I'd initially worried about, ended up paying 120 € total for replacing the main relay. The best part is they found the ECU wasn't defective at all, just wasn't getting proper power from that faulty relay. Once they swapped the relay, all those annoying symptoms disappeared completely. My M5's been running like a dream for months now, no more sluggish starts, no power issues, and all systems working perfectly. You know that anxiety you get when you're not sure if your car's going to start properly? Totally gone now. Seriously, it was such a relief that it wasn't anything more serious. Sometimes the simplest fix is the right one. Hope this helps give you a better idea of what you might be looking at cost-wise!

Thanks so much for the follow-up! I actually just got back from the mechanic today and you were spot on, it was the main relay causing all the trouble! Can't believe I was stressing about needing a whole new ECU. The repair cost was even less than what you paid. My mechanic had the part in stock and fixed it in about an hour. The car fired right up after the replacement, and that sluggish feeling is completely gone. All the warning lights cleared up too. Such a relief to have this sorted without breaking the bank. The car's running better than it has in months. Really appreciate you pointing me in the right direction, saved me from going down an expensive rabbit hole with the ECU. Sometimes the simplest answer really is the right one! Thanks again for your help. It's great to have my M3 back to its old self.