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Depollution can come on screen for a number of reasons,
Either the fluid in the polution tank is low, (this can be checked on diagnostics)
There is too much soot in the system ( also checked on dignostics)
The fuel filler cap is faulty ( diagnostics again)
Or you can drive for 10 miles over 50 mph without touching the brake pedal, this gets the system HOT and burns off the Soo