It sounds as though the turbo wastegate is opening because the control
system thinks things are going wrong and the turbo pressure is too high.
This happened on my 807 and the turbo had to be changed, but I believe it
can be sorted nowadays with a software download.  Someone who really knows
will probably be along to tell you soon...

Ron Robinson

Hi ,  "depollution faulty"  is being shown because the vehicle may be
emitting pollutants outside of man spec .Everything now is geared
towards  emissions.  (I.e.  an ignition misfire will bring this message
too ).This can be due to any number of things.IN your case and your
engine (2.0) 136 bhp dw10bted4 engine some of the most common problems
are

A) turbocharger itself.It has a repeat position sensor on it basically
what it does is tell the engine ecm the actual position of the turbo
vanes (hall effect)-These jam up/partially seize .New turbo as you
can't get them seperate.
B)Turbocharger control soleniod valve  .suffer from water ingress hence
loss of efficency.Early ones were poor anyway (have date stamp on them)
c) Actual vacuum control to turbo can be low .(Too many causes to
list!)

My bet is the turbo itself.But when you take it to the dealership im
sure the master tech will give you a 100% diagnoses ...

hope it gets fixed soon  mark