What was found
AFTER TAKEOFF CLIMBING OUT, STRONG DIRTY SOCK SMELL STARTED IN THE CABIN AND PROGRESSED INTO THE COCKPIT. DUE TO TOXIC FUMES, RETURNED FOR OVERWEIGHT LANDING. REMOVED AND REPLACED APU PER 26L31049005. COMPLIED WITH ECS DECONTAMINATION PER 26L00021004, NO DIRTY SOCK SMELL NOTED. HONEYWELL TEARDOWN REPORT COULD NOT CONFIRM SMELL IN CABIN - NO OIL LEAKS NOTED. AT TIME OF APU REPLACEMENT, THE OIL CAP WAS REPLACED AS WELL AS THE FLANGE AND SHAFT SEALS ON THE APU GENERATOR.