I had this today, and it was because there was a tear in the air hose leading to the pressure switch. It's a very easy fix that you can do yourself. Turn the power off at the mains (very important), open the machine up by taking off the lid (just unscrews at the back), and the pressure switch is a small circular white plastic object at the top, just behind the timer. The air hose comes in underneath on my model. It had a small tear in it so the air was leaking out. I trimmed the hose and reclamped it to the switch. The pressure switch is what the machine uses to tell how much water has come in, so if it doesn't work then it will throw an error. Took about half an hour all in.
David, July 2009