The filtered kitchen extract has got an in-built G3 filter, which protects the ductwork from grease of cooking fumes. If the filter has discoloured it needs changing.
To access the filter grid, slide it about 10mm to one side (towards the whole in the frame, if it is a proud filter housing). The grid will come out of the frame on the opposite end and can be taken out. In order to access the filter material, gently prise open the grid with a small flat screw driver. Replacement filter mats can be bought online (order code F-E3).