overfill error

Hello, Am having a lot of problems with our Elga Purelab Chorus. It is showing 'overfill alarm' on the screen. I have replaced the pressure sensor that is screwed into the base of the reservoir but it hasn't solved the problem. The vent that is screwed into the side of the reservoir gets saturated with water which limits the flow from the tap as air can't enter the reservoir. Am thinking the circuit board within the reservoir might be the problem. Any suggestions would be gratefully received.