BMW 3 Series Diesel Multiple System Failures, Help Needed

Hey there! I had a similar issue with my 2002 BMW 5 Series diesel (had some experience working on BMWs, mostly basic maintenance). The symptoms you're describing sound exactly like what I dealt with. Mine would take forever to start, felt sluggish, and systems would out of nowhere shut down. Turned out it was just a faulty main relay, not the ECU thankfully! The mechanic at my regular workshop diagnosed it quickly, and it was a relatively straightforward fix. Cost me about 120 Euro total, much better than replacing an ECU. Before I can help more specifically, which exact 3 Series model do you have (318d, 320d, etc.)? Also, what are the specific error codes you're getting? This would help confirm if you're dealing with the same issue I had.

Thanks for sharing your experience! My 320d has been showing these symptoms for about a week now. I just had it serviced last month at 143236 KM, and everything was fine until these issues started popping up. I forgot to mention earlier, but I'm also getting a battery warning light occasionally. I'm relieved to hear it might just be a relay issue rather than the ECU, that would definitely be easier on the wallet! I'll check on those error codes and get back to you. Did your relay problem come with any warning lights on the dashboard?

Hey again! Yes, definitely had warning lights with my 5 Series, including that pesky battery warning light you mentioned! Actually, that's one of the things that made my mechanic suspect the relay in the first place. Looking at what you've just shared, it sounds even more like what I experienced, especially since you mentioned it started suddenly after being fine post-service. Mine did the exact same thing, was running perfectly after a service, then bam, all these issues started showing up about two weeks later. The battery warning light is really interesting because that's exactly what confused me at first, I initially thought it was a battery issue and even had it tested, but the battery was perfectly fine. The main relay can cause these weird electrical gremlins because it's basically the middle- between your battery and various electrical systems. One quick thing you could try before heading to the mechanic, listen carefully when you turn the key to the first position (before starting). If you hear a weak or inconsistent clicking sound, that's another sign it might be the relay. That's what I noticed in mine just before I got it fixed. Let me know those error codes when you get them, I still have my old diagnostic report somewhere and could compare them. But honestly, it's sounding more and more like we had the same issue!

Thanks so much for all this detailed info! I just went out to check that clicking sound you mentioned, and sure enough, I'm hearing exactly what you described, it's more like a faint, inconsistent click rather than the usual solid click I'm used to. This is seriously relieving to hear that it might be something as simple as a relay. I just pulled the error codes: P0685 and P0687. These seem to be related to the main relay circuit, which totally fits with what you're describing. Think I'll go ahead and book it in with my mechanic tomorrow and specifically mention the relay issue. You've probably saved me a ton of worry and money here, I was already dreading having to replace the ECU! Will let you know how it goes. Thanks again for taking the time to help me troubleshoot this!