What was found
AIRCRAFT WAS GROUNDED: DURING TAXI OUT ANOTHER AIRCRAFT REPORTED FIRE / FLAMES COMING FROM THE APU EXHAUST. THE FIRE DEPARTMENT EXTINGUISHED FLAMES AND THE AIRCRAFT SAFELY RETURNED TO THE GATE. APU HAD BEEN SHUT DOWN AFTER ENGINE START SEVERAL MINUTES EARLIER. APU FUEL VALVE HAD FAILED TO CLOSE. THE FLIGHT CREW PRESSED APU FIRE PUSH BUTTON SWITCH AND PULLED APU FUEL CIRCUIT BREAKER TO NO AVAIL. NO ECAM RECIEVED.MAINTENANCE REPLACED THE FOLLOWING ITEMS: APU FUEL CONTROL PER AMM 49-32-11-400-003, APU FUEL VALVE ACTUATOR PER 28-29-51 AND THE APU MASTER SWITCH. A SATISFACTORY LEAK AND OPERATIONAL CHECK WAS ACCOMPLISHED.