Citroen DS3 IAT Sensor Fault

Those symptoms certainly point to a potential issue with the intake air temperature (IAT) sensor. It measures the temperature of the air entering your engine, which is crucial for the ECU to calculate the correct air-fuel mixture. If it's faulty, it can send incorrect readings to the ECU, leading to reduced engine power and rough running. Have you checked the wiring and connections to the IAT sensor for any signs of damage or corrosion?

Thanks. Can you explain how the ECU uses the IAT sensor data to manage the air-fuel mixture? I'm interested in the specific mechanism.

The ECU uses the IAT sensor reading as one of several inputs to determine the density of the incoming air. Colder air is denser, requiring more fuel for optimal combustion. If the IAT sensor sends a consistently high or low temperature reading, the ECU might inject too much or too little fuel. Further diagnostics are needed, but I'd recommend taking it to a workshop to ensure the diagnosis and repairs are done correctly.

I took it to a workshop as you suggested. They confirmed the intake air temperature sensor was defective and replaced it for 155€, which resolved the issues.