R Klasse Diesel Check Engine Light and Fuel Consumption Issue

That sounds familiar! I had a similar issue with my R Klasse (not sure of the year though) and those symptoms point towards an issue with the emission control system. The temperature sensor is a key exhaust component, so it might be the culprit. Have you noticed any unusual smells or sounds from the car exhaust?

No weird smells or sounds from the car exhaust that I've noticed. What exactly is an exhaust temperature sensor anyway? And what does it have to do with fuel?

The exhaust temperature sensor, or heat sensor, monitors the temperature of the exhaust gases. This is necessary because the ECU (Engine Control Unit, the car's computer) uses that information to optimize engine performance and ensure proper operation of the catalytic converter and other emission control devices. If there's a leak in the exhaust system, as you mentioned, it can throw off the sensor readings and cause all sorts of problems. I would say it will be best to take it to a workshop and have them check the temperature sensor and exhaust component.

Thanks! I took it to the workshop, and you were right. It was the exhaust temperature sensor. They found a small leak in the exhaust too. Cost me 265€ to fix, but at least it's running normally again.