As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER STARTING APU AND TURNING ON THE APU BLEED AIR VALVE, THE COCKPIT AND CABIN STARTED FILLING WITH SMOKE. SMOKE SEEMED TO STOP ACCUMULATING AFTER TURNING THE APU BLEED AIR VALVE BACK TO THE OFF POSITION///SIGN-OFF///C/W TC-68A-21-181006M ENVIRONMENTAL CONTROL SYSTEM BLEED DUCT BURN PROCEDURE USING LATEST REVISION POSTED ON COMPLY 365 AND REPLACED HEPA FILTER IAW CE680A AMM CH. 21-70-10. OPS CHECK GOOD.