This can be caused by the non return valve next to the drain pump impeller not sealing and allowing dirty water back into the machine when it is rinsing. To diagnose (You will have to be cautious as there are live wires and terminals) remove the left hand side panel , the one on your left as you are facing the door of the machine . (two screws are accessible when the door is open) You will see a blow molded series of pipes at the rear. The pipe at the back of this is connected to the drain hose( Grey ribbed pipe with a black rubber connector) follow this upside down "U" bend and the other opaque ribbed pipe is from the drain pump. Both sections should be full with water right up to the top of the "u" bend if the valve is working correctly. If this is full of water and the level is not falling the non return valve is OK. If the level in the right hand side of the "U" bend is falling the none return valve is not sealing and dirty water is running back into the machine. If you are not sure, Turn the machine on and run the prewash cycle. Wait for the machine to stop filling. Now end the cycle by pressing the "normal wash"and the "quick wash" buttons at the same time. This will then run the drain pump and water should be seen running round the "U" bend. When the pump stops the water should not run out of the right hand section of the "U". If it does this can happen for the following reasons. 1) the rubber has perished 2) There is dirt preventing the valve closing 3) The rubber valve has swollen and cannot flap back against the sealing face. To fix this you need to remove the debris filters from the machine. When these are removed you will see on the left hand side of the sump a plastic cover with a single screw in it. remove the screw and lift the cover. You will notice a strip of black rubber. Pull this out of its housing. It should consist of a rubber disk connected to a rectangular rubber plug. Check this for damage. If the face of the valve is damaged you can try putting it back the other way round so that the undamaged side is facing you, but first look into the hole that it came out of. If it is blocked with dirt clean it. You can soak it overnight with a solution of caustic soda or a drain cleaning product. Biological washing powder can also dissolve grease and grime if you do not have the others to hand. Clean the rubber valve by hand with washing up liquid. Once clean rinse and replace parts and test the machine again. If the valve seams to be in good condition but not sealing it is possible that it has swollen over time. This makes the overall length of the valve too long to seat correctly. To test if this is the problem put the valve back into it housing but dont push it all the way in, leave about 6mm of the rubber proud of the housing. Run the machine again but don't put the plastic cover back or replace the debris filter. You may have to adjust the amount you put the rubber back into it's housing until you get it to seal. When you succeed you will need to trim the rubber flush with the top of the hole it sits in. Replace the cover and test again. You should find this solves the problem. Good luck.