What was found
STARTED APU AFTER LANDING AND ESTABLISHED APU GENERATOR ON LINE. AFTER ARRIVING AT GATE THE APU BLEED VALVE WOULD NOT OPEN. GROUND POWER WAS HOOKED UP AND PUT ON ACFT. SWITCHED BACK TO APU GENERATOR AND IT WOULD NOT STAY ON LINE. ACFT GROUNDED. PERFORMED FIM TASK 49-31-00-810-809 FOR MX MESSAGE 49-31171. INITIAL EVAL CDU DISPLAY DOES NOT SHOW MAINT MESSAGE ON THE CURRENT STATUS PAGE, DETERMINES INTERMITTENT FAULT. PERFORMED TASK 49-11-00, APU OPS TEST AND NO FAULTS FOUND, APU OPERATES NORMALLY. REPLACED APU FUEL CONTROL UNIT, REF MM 49-31-11-400-801. OPS CHECK & LEAK CHECK GOOD.