As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT RETURNED TO PLATFORM AS A CONSEQUENCE OF DURING PUSH BACK, THE BLEED APU WITH 12 PSI INDICATION. THE ENGINES WERE STARTED WITH EXTERNAL PNEUMATIC PLANT IAW MM 49-50-00 AND THE APU REPORT WAS DEFERRED IAW MEL 49-7. AT MEX, THE CDU TEST WAS PERFORMED. THE FAULT CODE 49-52-191 WAS SHOWN AND THE PNEUMATIC APU WAS OUT OF SERVICE IAW MM 49-00-00. DURING OVERNIGHT CHECK, THE FIM PROCEDURE WAS PERFORMED IAW FIM 49-55 TASKS 806-807, TESTED CORRECT.