Koleos battery draining

Sounds like a defective battery sensor. Those symptoms – discharged battery, speedometer error, weak lights – are typical. Have you checked the battery's resting voltage with a multimeter? Also, get the error code read; it might give you a clue if it's a parasitic draw or a charging issue. A failing battery sensor can muck things up with the ECU and cause all sorts of electrical gremlins.

Thanks for the quick reply! I haven't checked the resting voltage myself, as I don't have a multimeter. But where is the battery sensor usually located, and is it easy to check without special tools? Would disconnecting it temporarily help diagnose the issue or make things worse?

The battery sensor is usually attached to the negative terminal of the battery. Visually, you'd be looking for a small module with a connector attached. Disconnecting it might throw another code, but it could help you see if the battery drain stops. However, given your description and lack of tools, I'd advise getting it checked by a workshop. They can properly diagnose it and replace the sensor if needed. Messing with the ECU without proper knowledge can cause more problems.

Okay, thanks a lot for the advice. I took it to the workshop, and you were spot on – it was indeed the battery sensor. The repair cost 315€, but at least the problem is sorted now!