BMW 5 Series ECU Problems, Cold Start Issues & Solutions

Hey there! I have some experience working on cars and faced a very similar issue with my 2004 BMW 3 Series (also had a 7 Series back then). When my car wouldn't start reliably, I also thought it was the ECU, but it turned out to be just loose battery cable connections! Took it to my trusted mechanic, who found corrosion on the terminals and poor contact. The fix was pretty straightforward, they cleaned the connections, tightened everything up, and applied some protection. Cost me about 155 Euro, but saved me from unnecessarily replacing expensive parts. Can you share more details about your car? Like: Current battery age? Are you hearing any clicking sounds when trying to start? Does the problem happen more after the car sits for a few days? This could help determine if you're dealing with the same issue or something else entirely.

Thanks for sharing your experience! My BMW has 154169 KM on the clock and just had its service last month. I actually hear a single click when trying to start, but no other sounds. The battery is fairly new, replaced it about 8 months ago. Interestingly, the problem does seem worse after the car sits for 2-3 days. I was worried it might be something major with the ECU, but your battery terminal story gives me hope it could be something simpler. Might get those connections checked first before diving into more expensive repairs.

Hey again! Wow, hearing about that single click really rings a bell with what I experienced on my 3 Series. After seeing your mileage and symptoms, I'm even more convinced it might be similar to what I dealt with. Even though your battery is relatively new (mine was only 6 months old when I had the issue), those terminal connections can get funky real quick. That single click you're hearing is exactly what my car was doing, it's typically the starter solenoid engaging but not getting enough juice to turn the engine over properly. Quick tip from my experience: try wiggling your battery cables when the car's off (carefully, of course!). If they feel even slightly loose, that's probably your culprit. My mechanic also found that even though my battery was new, the cable ends had some hidden corrosion creeping up inside the insulation, sneaky stuff that wasn't visible at first glance. Since it's worse after sitting, it really matches my situation. The connection probably deteriorates just enough during those idle days to cause problems. I'd definitely recommend getting those connections checked before worrying about the ECU. Could save you a bunch of money like it did for me! Let me know what you find out. I'm betting it'll be something similar to what I encountered.

Thanks so much for the detailed advice! You're really helping me feel less worried about this whole situation. I just went out and tried wiggling those battery cables like you suggested, and wouldn't you know it, there's definitely some movement in the negative terminal! That combined with the similar clicking sound and the way it acts up after sitting idle really makes me think you've nailed it. Going to call my mechanic tomorrow and have them check the connections first thing. Keeping my fingers crossed it's just a simple fix like yours was. The fact that you had almost identical symptoms with your 3 Series and it turned out to be something so straightforward is super reassuring. Will definitely update once I get it sorted out. Thanks again for taking the time to share your experience, might have just saved me from an unnecessary ECU replacement!