As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING APU START CREW ON RAMP REPORTED LARGE VOLUME OF SMOKE, APU START ABORTED AND FAULT LIGHT OBSERVED ON. GROUNDED - YES. REPLACED APU IGNITION UNIT REF B737-800 AMM 49-41-31. ALSO REPLACED IGNITOR PLUG REF AMM 49-41-51. OPS CK GOOD. INDICATIONS NORMAL.