Vaillant ecoMax boiler keeps showing F.23

The boiler keeps going to F.23, the pump has been changed for a new and the diverter valve has been changed. The boiler has been taken apart and there is no blockages,in major pipes and there's none. The out flow pipe seems more hotter than the return pipe.The present engineer thanks it is the fault of the 'Auto return valve' just below the counter, and I am waiting for it to be change. Also all I have check all radiators for air. If anyone had this problem and sorted it please leave a comment how they sorted it out. So I can past this on to an engineer. Thanks Karen