As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
HEAVY GREY SMOKE FILLED THE CABIN WHILE APU OPERATING DURING PRE-FLIGHT. OPENED ACCESS & REMOVED APU PN 3800722-1 & SN P-677 FROM ACFT. PERFORMED TEAR DOWN & BUILD UP PROCEDURES FROM REMOVED TO REPAIR APU. INSTALLED REPAIRED APU, SAME PN, SN P-669 IAW CH 49-10-00 OF THE MM. OPERATIONS & LEAK CHECKED GOOD. CLOSED APU ACCESS PANEL & CONTAINMENT BOX PANELS & SEALED PANELS AS NEEDED IAW MM 49-10-00.