As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PDX - FLIGHT CREW NOTED SMOKE COMING FROM AIR CONDITIONING VENTS. AIRCRAFT WAS AT THE GATE FOR 45 MINUTES WITH NO PROBLEM, THEN WHEN BOARDING, FLIGHT CREW NOTICED SMOKE BEGAN SLOWLY (COCKPIT OFF CABIN ON). APU REPLACED I/A/W MAINTENANCE MANUAL AND APU OPS CHECKED GOOD.