Over time, minerals and contaminates can be left behind by the evaporation process. This can also be caused by testing the sensor using a liquid other than clear water. Replacement sensors can also be ordered. These sensors contain configuration switches that allow the sensor to operate properly for the application. This is what gives the unit flexibility.
Improper settings can also be the cause of unexpected operation.
The index mark on the rotary switch is the small arrow at a right angle to the screwdriver slot, not the screwdriver slot itself. These may be going toward or away from the legends. I've just tested my sensor but don't want to wait for the delay-off timer. This will clear the delay-off timer. On all control cables the White lead is digital ground.