BMW E39 Won't Start: Key Symbol Light Points to Sensor Issue

Hey there! I had something very similar with my BMW 523d (2002) about a year ago. I have some experience working on cars, but this was beyond my DIY skills. Like yours, my car wouldn't start, and that annoying key symbol was lit up. Initially thought it was just the key battery, but nope! Turned out the control unit was totally shot. My mechanic diagnosed it and replaced the affected unit. Cost me around 1000 Euro, and they had the car for two days. Before I can give you more specific advice though, could you tell me if you're getting any other warning lights? Also, have you tried using your spare key? Would help to know what exact error codes you're getting from the diagnostic too. For reference, I also own an E39 525i and never had this issue with that one. Might be more common in the diesel variants.

Thanks for sharing your experience! I've got 142882 KM on my 520d, and actually had my last service just 3 months ago. I checked with the spare key already, same issue. Getting error codes EWS-2 and EWS-3 from my scanner. No other warning lights on the dash, just that frustrating key symbol. Really hoping it's not the control unit as that sounds pretty expensive! Been trying to find a good repair shop that knows their way around these old BMWs, especially the diesel models. Anyone know a reliable mechanic who's dealt with these specific EWS errors before?

Hey again! Thanks for the additional details about your 520d. Man, those EWS-2 and EWS-3 codes bring back some not-so-fond memories. I actually got the exact same error codes on my 523d before the whole control unit drama. Look, I hate to say it, but after seeing you've tried the spare key and you're getting those specific EWS codes, I'm pretty convinced you're dealing with the same control unit issue I had. The fact that both keys aren't working pretty much rules out a simple key problem. My car had similar mileage too (around 150k km) when this happened. Initially, I wasted time and money trying to fix the Hall sensor because, like you, I was hoping for a cheaper solution. Big mistake. Should've gone straight for the control unit diagnosis. Quick tip though, before you commit to replacing the whole unit, make sure your mechanic checks the wiring connections to the control unit. Sometimes there can be corroded contacts that cause similar symptoms. Would've saved me some cash if my mechanic had checked that first! I know it's not what you wanted to hear, but at least you might save yourself some time and frustration by targeting the real issue straight away. If you need any more specifics about the repair process, just let me know.

Thanks for the detailed insight, definitely not what I wanted to hear about the control unit, but better to know the likely culprit now than waste time on other fixes! Really appreciate the tip about checking the wiring connections first. I've called a few workshops today and found one that specifically mentioned experience with EWS issues in older BMWs. Got an appointment scheduled for tomorrow morning. Will have them check those connections before jumping straight to replacing the whole unit. I'll update this thread once I know what the actual problem was, might help others with similar issues. Thanks again for sharing your experience, saved me from going down the Hall sensor rabbit hole!