Overheating problem and illuminated check engine light on 2003 BMW 3 Series - Possible problem with the coolant temperature sensor?

Hey, I know the problem a bit, I had a similar problem with my 2005 BMW 5 Series. Well, I'm not a car expert, but I do have some experience with cars. My sensor failed back then, but in my case it was the engine control unit that was faulty, it was crazy expensive, it cost me 1000 euros. I had to have it changed in the garage and since then everything has been running great again. Maybe you can give me a few more details about your car, that would help me to be more precise. For example, what's the mileage like and when was the last time you had an inspection?

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Hey, thanks for the quick reply. My BMW already has 256981 km behind it. The last service was recently, but the problem just hasn't gone away. Hope this helps you.

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Hey, thanks for the info. Based on the mileage, I would say that it could be a wear problem. I had the same problem with my 5 Series. If you've already been to the garage and the problem is still there, then they may have missed something. In your case, I would suggest you go back to a garage and ask specifically about the sensor and the engine control unit. Tell them about your suspicions and that you've already had a major repair that didn't solve the problem. Any good mechanic should then be able to systematically search for the fault. In my case, the engine control unit was faulty, but there are several reasons for this. Have a look, because at this mileage it is also possible that other parts are no longer working properly. It could also be the water pump or the thermostat. But the coolant temperature sensor is also a usual suspect. Maybe the problem is more complex than you think. Hope this helps and good luck getting it sorted soon!

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Thank you for the detailed reply and the advice. I will take your advice and go back to another garage, give them all the details and specifically have the engine control unit and coolant sensor checked. I hope that it is not a very expensive fault and that it can be repaired soon. Many thanks again!

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Are you sure that the vehicle is not really overheating? In this case, you should not move the car to prevent engine damage

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So I would also advise you to have the whole thing checked before you continue driving as already mentioned.

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Replaced alternator and battery multiple times. Replaced battery wires and the red box on top of battery. Got it coded with the Carly app. Car does not charge the battery. It is not an alternator or battery issue since I’ve bent through several thinking they were bad at first. I’ve checked the alternator wire and I get a power and a ground from just the single pin is that normal?

It sounds like you've done a lot of troubleshooting already. If you're getting both power and ground from the single pin on the alternator, that's not normal and could indicate a short circuit somewhere. I'd recommend checking the wiring harness for any signs of damage or corrosion. Also, check the voltage regulator if it's separate from your alternator. If you're still having trouble, it might be best to have a professional mechanic take a look. Please update us on your progress.

If my coolant temperature sensor bad could my fan not turn on in the e90.

Yes, a faulty coolant temperature sensor could cause the fan not to turn on in your BMW E90. The sensor provides information to the engine control unit about the engine's temperature. If the sensor is faulty, it may not send the correct signal to turn the fan on when needed. I would recommend starting with checking the coolant level and the fuse for the fan. If those are fine, you might want to consider having the coolant temperature sensor checked or replaced. Please let us know how it goes.