The only possibilities are a sensor with a mineral film, in general it takes a few hours for vinegar to remove this. If the sensor is scratched or scuffed this can also be a cause.
The other possibility is the sensor is shorted in some way, a damaged wire, water entering the sensor, etc. In this case the controller should be sent in and the sensor will be replaced, if it is less than 2 years old there would be no charge.