What was found
APU WILL NOT START. ACFT GROUNDED. APU BITE COMPLETED IAW FIM 49-60 TASK 80, NO CURRENT FAULTS LOGGED. FAULT HISTORY SHOWS FAULTS FROM OVER 60 DAYS AGO. APU START SWITCH S248 REMOVED AND REPLACED IAW WDM 49-6211. NO HELP, AND FAULT 49-41181 IGNITION UNIT CIRCUIT SHOWS OPEN CIRCUIT LOGGED. ECU REMOVED AND REPLACED IAW AMM 49-61-12, NO HELP AND ORIGINAL INSTALLED. POWER CHECK DONE AT CONNECTOR P13 IAW WDM 49-41-11. POWER IS PRESENT. IGNITION UNIT SHOWS NO OUTPUT. REMOVED AND REPLACED IGNITION UNIT IAW AMM 49-41-31. REMOVED AND REPLACED IGNITOR PLUG IAW AMM 49-41-51. OPS CHECKED GOOD. APU STARTS ON 1ST ATTEMPT. NO FURTHER DEFECTS NOTED.