As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING ENGINE START, CREW REPORTED SMOKE HAZE IN CABIN WITH A BURNING ODOR. RETURNED TO GATE. MAINTENANCE RAN BOTH PACKS HOT AND COLD IN ALL BLEED CONFIGURATIONS AND PERFORMED OPS CHECKS IAW AMM. OPS CHECK GOOD. COULD NOT DUPLICATE DISCREPANCY. ACFT RELEASED FOR SERVICE.