Low Fresh Water Level Indicator

The fresh water level indicator on my EC51 control panel shows red and it emits the audible beep even when the tank is full. This doesn't happen all the time but it happens intermittently. Sometimes it can be showing full and then will suddenly flash red empty and beep. I have done some research and what the problem maybe is that the lowest stud in the water tank is dirty. If this is the case does anyone know how to clean it? Any other suggestions as to what the cause maybe would also be appreciated.

Buy some citric acid from Amazon. I used 500 grams in about 25% tank using some hot water to aid dissolving. I left that for a day and then topped up the tank, leaving that for 2 or 3 days. I then drained down and refilled twice.
It works for me and lasts for a year or so.
I did not pump the citric acid into the hot tank.

Okay thanks Ron I'll give it a try.