What was found
(CAN) DURING TAXI, IT WAS OBSERVED THAT THE PRIMARY FLIGHT DISPLAY SCREEN WENT BLANK. A BURNING SMELL WAS SENSED. THE AIRCRAFT RETURNED TO THE GATE WITHOUT INCIDENT . THE PFD UNIT WAS REPLACED AND THE AIRCRAFT WAS RELEASED. DURING TAXI, THE PFD SCREEN WENT BLANK AGAIN. THE AIRCRAFT RETURNED TO THE GATE. UPON FURTHER INVESTIGATION, WATER AND MOISTURE WAS FOUND WHICH CAUSED THE UNIT TO BECOME UNSERVICEABLE. A LEAK WAS FOUND COMING FROM THE WIPER MOTOR AREA AND THE SEALANT HAD DETERIORATED. THE AREA WAS CLEANED OF ALL THE OLD SEALANT AND REPLACED. A SERVICEABLE PFD WAS INSTALLED. THE AIRCRAFT WAS RELEASED WITH NO FURTHER INCIDENT DURING SUBSQUENT FLIGHT.