BMW M4 ECU Problems: System Failures & Power Loss Help

Hey there! Had a similar issue with my 2018 BMW M3 last year. I've got some experience working on cars, but nothing too advanced. The symptoms you're describing sound exactly like what I dealt with, those frustrating unusual shutdowns and slow starts were driving me crazy! Initially thought it was the ECU too, but turned out to be a faulty main relay. My trusted mechanic diagnosed and replaced it (120 Euro for the whole job), and the car's been running perfectly since then. Could you share more details about when these issues typically occur? Like, does it happen more when the engine's hot or cold? Also, what's that error code you mentioned? That would help narrow things down. By the way, had a good with an M5 who experienced similar symptoms, but his turned out to be something completely different, so it's really important to get a proper diagnosis.

Thanks for sharing your experience! Mine's at 195648 KM, last serviced about two months back. The issues typically happen when the engine's already warm, usually after about 15-20 minutes of driving. The error code is P0685, which points to the ECU power relay circuit. I'm a bit relieved hearing about your relay fix being relatively straightforward and not too expensive. Was your mechanic a BMW specialist? I've been hesitant about taking it to the dealership since my warranty just expired, and their diagnostic fees alone can be pretty steep. Did you notice any warning lights before the full shutdowns started happening?

Hey again! Yeah, my mechanic was actually a BMW specialist, but not from the dealership, found him through word of mouth from other BMW owners. Super knowledgeable guy who didn't charge an arm and a leg. With my M3, I did notice the check engine light flickering a few times before the shutdowns started happening. Mine also showed the P0685 code, that's actually what made me suspect the ECU initially. The symptoms happening when warm are really similar to what I experienced too. My car had about 180,000 KM when this happened. Just to add to my earlier reply about the relay fix, the repair actually ended up being even simpler than I expected. The mechanic tested the circuit and found the relay was intermittently failing when it got hot, which explains why it mostly happened after driving for a while. What I learned from this whole experience is that these symptoms can be super misleading. The main relay failing can mimic ECU problems because it's basically controlling the power supply to it. If you're seeing the same P0685 code, I'd bet good money you're dealing with the same issue I had. Definitely worth checking the relay before worrying about a full ECU replacement. Have you noticed any other electrical issues, like unusual warning lights or instrument cluster glitches?

Thanks so much for the detailed feedback! It's such a relief to hear that you had the exact same error code and symptoms. No other electrical issues or instrument cluster problems so far, just those annoying shutdowns. I've actually got a recommendation for a BMW specialist through my car club, and after hearing your experience, I'm going to give them a call tomorrow. Really hoping it's just the relay like in your case, sounds way better than shelling out for a new ECU! Will definitely have them check the relay first and run a full diagnostic. The similar mileage and symptoms make me pretty confident now. Thanks again for sharing your experience, you've saved me a lot of worry and potentially a lot of money too! Will update once I get it sorted.