As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DEPARTED, SHUT DOWN APU, TURNED OFF BLEEDS, THEN SMOKE APPEARED IN THE COCKPIT. ALSO SMELL OF BURNT OIL. RETURNED FOR LANDING. REMOVED FAILED ACM PN:3471A020000 SN 00734 AND INSTALLED CUSTOMER SUPPLIED MODIFIED ACM PN: 3471A020000 SN: 00430. PERFORMED SATISFACTORY OPS/LEAK CHECKS WITH NO DEFECTS NOTED. REF BD-100 AMM 24-52-13