Likely scum on the sensor, if you leave them full. I had this once and now fill up the ballast with a couple gallons of strong bleach, a touch of dish soap, and tap water at fall layup. I drive the boat on the trailer to the gas station to top off with fuel, and then pump out and add a couple gallons of RV anti-freeze. No problems since. Just have a small tube connected to a funnel and stick in the overflow. Do this outside of the boat and be patient. It is easier on a step ladder so you don't have to hold it higher than shoulder level for so long. It takes a while - be patient.
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