Citroen C5 Transmission Control Unit Fault

Based on your symptoms regarding your 2005 Citroen C5 diesel, a defective transmission control unit appears to be the most probable cause. The issues strongly suggest electronic malfunction. Has the diagnostic error code been specifically identified? Understanding the exact code would further solidify the diagnosis. Have you inspected the wiring and connections to the transmission control unit for any signs of damage or corrosion? These are crucial diagnostic steps.

I retrieved the error code, which indicates a malfunction in the communication between the gearbox ECU and other control units. I visually inspected the wiring harness for obvious damage, but did not find anything. What should I do next?

The communication malfunction combined with your other symptoms certainly points to a failing control unit. Considering the age of your Citroen C5, and the mileage, these failures are expected for electronic components. Although further detailed electrical testing could be performed, it might be more economical to have a qualified workshop evaluate the unit. It might also be necessary to replace the control unit, and reprogramming will likely resolve the issue. Expect a repair cost around 1000€.

Based on your advice, I took the Citroen C5 to a local repair shop where they confirmed the diagnosis of a defective transmission control unit. They replaced the unit, performed the necessary reprogramming, and the vehicle is now functioning correctly. The total cost was indeed close to 1000€. Thanks for your expert help.