Frigidaire and Electrolux dishwasher .20 or I20 error code is directly connected to the water level or pressure sensor located in the bottom of the tub assembly. What the manufacture has found is that the sensor is getting plugged with debris from the wash cycle. After removing the two filters located just below the bottom spray arm you will notice a small round hole located on the left side of the tub. This is the water level sensor hole and has no protection from items entering into the sensor itself.
What you will need to correct this problem is the part # 5304504077 sensor kit with instructions. In the