Citroen C4 Engine Control Malfunction

It is possible. Have you checked the voltage across the battery terminals while attempting to start the engine? Also, are there any unusual lights on the dashboard, specifically the engine light? A fault code read might provide more insight into the ECU issue or potential fuel injection problems.

I have not measured the voltage under load, no. Could a faulty ECU prevent the engine from starting, even with good battery voltage?

Yes, a faulty ECU could definitely prevent starting even with good voltage. The ECU controls essential functions like fuel injection and ignition. However, given the symptom and mileage, I would advise you to have the cable connections and the ECU checked at a workshop.

Thank you for the advice. The problem was indeed poor battery cable connections. Repair cost was 155€.