This happens to me all the time. I've had this machine for four years now and it went backwards and forwards between the technicians' workshop and my home (during the first two years of the guarantee period, anyway) and they've replaced the display and the processor board God knows how many times. This fixes it for about a fortnight and then the dreaded AL 99 message returns. I keep the machine running by opening the door, pressing the power button to get rid of the AL 99 message, slam the door shut again (not so hard as to break all the dishes inside!!) and touch the power button again to see if it has disappeared. If not, I repeat the process again and again until it works. Then I choose the programme and press the start button quickly before it changes its mind. Once the machine is running, it doesn't matter whether the AL99 reappears or not, as the programme will continue until the end.
Guzzidave, February 2009