How can I get rid of Alarm AL5 on Zanussi DES 959 dishwasher?

My Zanussi DES 959 dishwasher has not been used for quite a while. I have now reconnected it and started to use it again, but it comes up with the Alarm AL5, although it fills up with water and there does not seem an obvious problem. I have checked taps, water supply, inlet hose, etc. but cannot find any fault. The dishwasher continues its washing cycle, as soon as I turn the power off and on again. (This happens about 3 times during each washing programme). Can anybody help?