He wants you to replace the entire manifold?
Thats the Part number you are stating - quite a task, so since the code is showing - the system is telling you it's stuck open, which is natural for its age...too much gunk in the manifold affects the vanes and so naturally they'll stick open due to the goo that collects.
So, the Mondeo menace rears its ugly head again.
What steps were taken to arrive to this "Doctor we have to do this this way" moment?
Usually taking off the manifold and dumping some Seafoam or Carb Cleaner down into it so it can sit and work - was that ever tried?
To help - the "YS" part number is a drop in repalcement for the manifold itself - it has moving vanes inside to help "swirl" the air in an effort to make turbulence so that the injectors spray can be atomized (fuel droplets to fumes) and make it burn better.
So, you have a year in which these "Vanes" are used.
Ok, replace the manifold, but what that does - doesn't fix what's makes them stick. - it could be a motor actuator - and or a vacuum port switch that feeds the motor manifold vacuum to move those vanes.
IT's why the investigation goes deeper than a simple replace this to fix that - because then the problem will possibly carry over from the parts taken off the older manifold to refit on the new one. Those parts are or could possibly be - where that problem really is.