BMW 3 Diesel Glow Plug Control Module Problem & Fixes

Hey there! I had a really similar issue with my 2012 BMW 5 diesel (these problems seem common in BMWs, I also considered the 3 series back then). I have some experience working on cars, but nothing too advanced. My symptoms matched yours exactly, preheating light stuck on, rough cold starts, and that annoying engine light. My workshop found it was just a bad connection between the control module and the glow plugs. The cable harness had some corrosion. Total fix was 95 Euro, mostly labor costs for cleaning and securing the connections properly. No need to replace the whole control unit in my case. But before jumping to conclusions, could you share what specific fault codes you're getting? Also, how many miles/kilometers are on your engine? This would help determine if it's just a connection issue like mine or something more serious. Quick tip: Don't wait too long to fix it. I drove around with the issue for a couple weeks and definitely noticed worse fuel economy and performance.

Thanks for sharing your experience! My car has just 20466 KM on the clock and had its latest service two months ago. Your feedback is really helpful, I've noticed the same symptoms with fuel economy dropping. I'll definitely get it looked at ASAP and have them check the connections first before replacing any parts. Did you notice if the smoke got worse in cold weather? I'm seeing quite a bit of smoke, especially during morning starts.

Yeah, I totally get what you're experiencing with the smoke! With my 2012 BMW 5, the smoke was definitely worse during cold starts. I remember being pretty embarrassed about all the smoke coming out, especially in the morning. It was like having a mobile fog machine! Given that your car only has 20466 KM, I'd say you're in a better position than I was, mine had about 90000 KM when these issues started. The low mileage on yours actually makes me more confident it could be just connection issues like mine was. Speaking of cold starts, did you notice the smoke is kind of bluish-gray? That's exactly what I had. It got way better after fixing those corroded connections. The morning smoke show completely disappeared within a day or two after the repair. One thing I didn't mention in my first reply, I also noticed the car would sometimes hesitate a bit before properly accelerating, especially when the engine wasn't fully warmed up. Did you experience anything like that? If so, it's another sign pointing to the connection issue rather than a failed control unit. Just keep in mind though, even though my fix was simple, it's still worth getting it checked soon. These BMW diesels can get pretty grumpy if glow plug issues aren't sorted out promptly!

Thanks again for all this detailed info! Yes, that's exactly what I'm seeing, bluish-gray smoke and that hesitation during acceleration you mentioned! Actually feeling quite relieved now. My symptoms match yours perfectly, and since my car has way fewer miles, I'm hopeful it's just those connections like in your case. I've already called my mechanic and booked it in for tomorrow morning. Will definitely have them check those connections first before replacing anything expensive. Really appreciate you taking the time to share all this, it's given me a much better idea of what to expect. I'll let you know how it turns out!