AO2 seems to be an overheat warning.
I fixed mine as follows.
Power off the appliance.Take the lid off,lower front below door 2 screws and 2 clips underneath.remove sides,turn unit upside down remove 4 base plate screws and you are into the mechanism.In the back left corner under the water out pipe you will see a nut with two wires clipped to it.One comes from the heater element.Pull off the wires and with a spanner undo the nut.It needs very little force as its only a small thread to hold it to the underside of the main compartment.Replace this part its a 72 degree thermal cutout cost 8.50GBP.Put back together is reverse of above strip down.
Now mine ran once and then repeated AO2.If this happens dont worry unclip the heater element from its rear bracket place something under the element and bend it down towards the floor a couple of degrees.Then reclip into place.What this does is it brings the heater element closer to the thermal cutout causing it to activate sooner and before the overheat sensors under the compartments middle cause AO2 to show.If you are squeamish about bending the element then make a brass element carrier that sits on the bottom of the compartment above the thermal cutout.
Rob Cave, February 2006
