I had the same problem, you have to replace the door lock. The door lock has a micro switch within that fails. It's quite straight forward to change yourself, just go carefully. You have to remove all the torc screws in the door, the unit is held in place with a plastic clip and there are two wires to remove, I got the part from ukwhitegoods.co.uk.part number 697690139 at £13.24 inc VAT , they will need the serial number to ensure this is the correct version of the model as it has change during its manufacturing life.
Andy.
September 2006