Wrong diognosis

We took vehicle for a service and we brought to their attention that the vehicle loses power they then gave the vehicle back to us (no charge on the no power part) saying they could not fault the vehicle. We then brought the vehicle back to us after it almost broke down the same day they took it from us . They have after a long time confirmed that the vehicle needs an engine control unit and the oxygen sensor. It took very long time. We are not happy with the service