In Frigidaire and Electrolux dishwashers, the .20 and/or I20 error messages appear when water level in the machine deviates from normal due to improper functioning of the pressure sensor. Most commonly, it happens when the sensor takes on food particles and debris during wash cycles.
Remove the two filters below the bottom spray arm. There will be a round hole on the left-hand side of the chamber. The pressure sensor is installed in the hole. Please, refer to Part # 5304504077 sensor kit: there will be a new pressure sensor and filter cover. Installing the sensor is not quite so hard to do, but if you are not sure, you can use professional help. In any event, please, check the sensor after each cycle and keep the filter clean. The error will go away.