Mini Roadster Diesel Engine Problem

That sounds like a troublesome situation indeed! The check engine light illuminating, coupled with reduced engine power and increased fuel consumption, strongly suggests an issue within the emission control system of your Mini Roadster. The exhaust sensor plays a crucial role in monitoring the exhaust gas temperature, which the engine management system uses to optimize combustion and minimize harmful emissions. A faulty sensor can indeed lead to the symptoms you describe. Have you noticed any unusual noises or smells coming from the exhaust system? Furthermore, has the temperature fluctuation been consistent, or does it vary depending on driving conditions?

No unusual noises or smells that I've noticed. How would temperature fluctuation affect it?

Think of it like this: your engine is a complex chemical reaction. The exhaust sensor is like a thermostat, ensuring the reaction happens at the optimal temperature. If the temperature fluctuates wildly, the engine management system gets confused and compensates by reducing power and increasing fuel. Given your symptoms and suspicion of a leak, it's highly probable that the exhaust sensor is indeed the culprit. It would be prudent to have a qualified mechanic diagnose the system to pinpoint the exact location of the leak and confirm the sensor's functionality. Sensor replacement will likely be necessary.

Thanks for the advice. The workshop confirmed it was a faulty exhaust temperature sensor, likely caused by a small leak. Sensor replacement cost me 265€ and the engine management light is now off.