BMW Z3 Check Engine Light: Water Sensor Issues & Fixes

Hey there! I've got some experience with BMWs as I own a 2003 BMW Z4 (and previously had a 3 series). I'd say I have low experience, mostly just basic maintenance stuff. Had a similar issue last month where my check engine light came on and the engine was running pretty rough. Turned out to be a faulty cable connection to the water temp sensor. My mechanic replaced the wiring harness and connector, cost me about 95 Euro total. Car's been running smooth since then. Before you move forward with repairs, could you share: How many miles/kilometers on your Z3? Are you noticing any other symptoms besides the rough running? Did you get the specific fault code from the diagnostic? This info would help me tell if your issue is identical to what I dealt with.

Thanks for your detailed response! I'm at 62964 KM and just had my service done recently. The only other symptom I've noticed is slightly higher fuel consumption lately. The diagnostic showed fault code P0117, which points to the coolant temperature sensor circuit. Your experience with the wiring harness is really helpful, sounds pretty similar to what I'm dealing with. I'm relieved to hear it might not be a super expensive fix. Guess I'll get it checked out by a mechanic sooner rather than later.

Hey again! Thanks for sharing those details about your Z3. Yeah, that P0117 code is exactly what I had with my Z4, definitely sounds like we're dealing with the same issue. The increased fuel consumption makes total sense too because when that sensor isn't working right, the engine computer can't optimize the fuel mixture properly. Just to give you a heads up from my experience, when my mechanic dug deeper into my car, they found that the wiring had gotten a bit corroded where it connects to the sensor. The harness replacement fixed it completely, and my fuel economy went back to normal within a couple of tanks. Since your mileage is pretty similar to what mine was when this happened (I was at about 65000 KM), it's probably just normal wear and tear on the electrical connections. These BMWs are pretty solid cars otherwise, but they do have these little electrical quirks that pop up. Quick tip: if you're taking it to a mechanic, ask them to check the connector housing too, not just the wiring. In my case, there was a tiny crack in the plastic that was letting moisture in. Getting that replaced as part of the job probably helped prevent the issue from coming back. Let me know how it goes when you get it fixed!

Thanks so much for all the detailed advice! Really appreciate you sharing your experience, especially about the connector housing, definitely gonna mention that to my mechanic when I take it in. It's such a relief to hear this is a relatively simple fix and not something major with the engine. Will probably book it in next week to get it sorted before it potentially causes any other issues. I'll make sure to get both the wiring harness and connector housing checked out like you suggested. Thanks again for walking me through this, it's been super helpful having someone who's dealt with the exact same problem. I'll update once it's fixed!