What was found
APU FAILED TO START. FAULT LIGHT AND LOW OIL PRESSURE LIGHT ARE ON. GROUNDED - YES. HAD MAINTENANCE MESSAGE " NO FLAME SHUTDOWN". PERFORMED B737-9 IFIM 49-40-00-810-804. VISUALLY INSPECTED APU AND FOUND RED BUTTON ON LUBE FILTER CASE EXTENDED. REMOVED AND REPLACED APU OIL LUBE FILTER AND APU OIL STARTER - GENERATOR FILTER PER B737-9 AMM 49-91-13. INSPECTED APU IGNITER PLUG AND FOUND IT WORN. REMOVED AND REPLACED APU IGNITER PLUG PER AMM 49-41-51, NO HELP. REMOVED AND REPLACED APU IGNITION UNIT PER AMM 49-41-31. OPS CHECK GOOD. APU OPERATING NORMAL.