I am answering my own fault, because I found the problem and it may help others. Firstly, the problem occurred each time after a wash cycle. I had assumed it was at the end, because I was never present when it occurred. On this particular day I pulled up a stool and decided to observe the dishwasher during a cycle, to get to the exact symptoms. Each time after a wash it filled with rinse water and that is where the problem occurred. It was a U1 error indicating drain/fill problem. At this point I opened the door and found cold, frothy water. Immediately I realised what the problem was. I had a detergent problem and the excess froth, was causing a sensor error. I drained the machine and ran up to 5 rinse cycles to clear the excess froth. I had found out later my wife had changed detergents. The new detergent had a rinse additive which I believe was re-acting with mine. In addition, she was using tablets, not powder and these particular tablets from this brand were not dissolving completely. This caused excess detergent to mix with the rinse-aid during rinse causing a reaction of froth. I have changed back to my usual powder and have had no further problems.
Johnny
June 2007