E90 indicates a communications error between the user interface board (control panel) and the main circuit board. If you take the top off the machine (power off first!), then stand in front of it and look at the front left you'll see the main board and a blue multi-stranded cable coming out of it and going into the control panel. 99 times out 100 this is the culprit, if there's a poor connection at either end then this generates E90. You need to remove the control panel (screws behind soap drawer and plastic clips along the top), disconnect the blue cable, give the connector a bit of a squeeze with your pliers to tighten the connection, do the same to the other end on the main board, refit everything and away you go.
