As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PHX - AIR TURN BACK - FLT 0001 - ON TAKEOFF FROM PHX EN ROUTE FOR ORD FUMES WERE NOTICED IN MID-CABIN, TURNED BACK TO PHX AND LANDED WITHOUT FURTHER EVENT. MAINTENANCE PERFORMED POWER RUN AT ALL SETTINGS ISOLATING THE NO.1 AND NO. 2 ENGINES BLEEDS AND APU BLEED, UPON INSPECTION FOUND RE-CIRCULATION FAN FILTERS DIRTY. REMOVED AND REPLACED BOTH FILTERS.