What was found
WHILE TRYING TO START THE APU THE CREW NOTED A STRONG FUEL ODOR AFTER TGE APU WAS STARTED. NOTE: IT IS DIFFICULT TO START THE APU, HAVE TO HOLD SWITCH UNTIL APU IS AT 100 PERCENT. MAINTENANCE REMOVED AND REPLACED THE APU EXTINGUISHER SWITCH, PERFORMED A OPERATIONAL CHECK, NO FUMES NOTED, AND, THE AIRCRAFT WAS APPROVED FOR RETURN TO SERVICE.