Koleos diesel fuel filter heater issue

Sounds like your fuel filter heater might indeed be the culprit. A defective cable or plug could disrupt the heater's operation. When the fuel isn't properly heated, especially in colder weather, it can affect fuel efficiency and engine performance. The ECU probably detected the fault, hence the error code. Have you checked the connection to the fuel filter? Any visible corrosion or loose wires?

I haven't really checked it myself, to be honest. I'm not very technical. Where exactly is the fuel filter located? And what does ECU mean in this context?

The fuel filter's generally located in the fuel line, often near the fuel tank or engine compartment. Check your car's manual for the exact location. ECU stands for Engine Control Unit; it's basically the car's computer. Given the symptoms and error code, and your lack of experience, it's probably best to get it checked by a workshop. A faulty fuel filter heater can definitely cause the problems you're describing.

Thanks for the advice! Took it to a shop and you were spot on. Turns out it was a defective connection to the fuel filter heater, just like you said. They replaced the connector and the error is gone, engine's running smooth again. Cost me 95€, but worth it to get rid of the headache!