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Motor & pump replacement parts for FRIGIDAIRE FDB520RHS0 DISHWASHER | AppliancePartsPros.com
Here is the tech sheet.
Check to see if the wash motor is getting 120 volts when it should be washing.
If it is and is not running or making any noise then it is toast.
If there is no voltage then the control board is probably shot.
You could try the water/service test.
If the motor runs then the board would be at the top of the list.