Engine fault message

Hi I have a Citroën ds4 2017. When I drive and especially used a lot of gas or on the a38 a message beeps and says engine fault repair needed this is then followed by eco activated and the eco light flashes for a couple of seconds. This comes up every few seconds until I restart the car. No lights remain on.

The car has been to the garage and we had a mobile mechanic, they have done a code reader and nothing can be found. The mechanic believes it’s a sensor fault and I should speak to a Citroën specialist.