What was found
COLD SOAK START(CSS) WAS INCOMPLETE. FIRST START ATTEMP FL390. FAULT LIGHT ILLUMINATED, WENT OUT AFTER 4 MINUTES. SECOND START ATTEMPT FL36.3 UNSUCCESSFUL WITH FAULT LIGHT ILLUMINATION. WENT OUT AT 4-5 MINUTE MARK. BUT ACFT HAD DESCENDED BELOW FL290. SUBSEQUENT ATTEMPTS ABANDONED. NOTE APU STARTED NORMALLY ON TAXI IN. R & R APU FUEL NOZZLES IAW 49-31-14 PG BLK 401, LEAK AND OPS CHECKED GOOD. COMBUSTION PLENUM DRAIN PLUG FOUND LOOSE. TIGHTENED AND SAFETIED PLUG.