What was found
AIRCRAFT WAS GROUNDED: ABOUT 5 MINUTES AFTER ENGINE START CABIN WAS NOT COOLING DOWN. WE SWITCHED TO MAX COOLING FROM THE APU. ABOUT 30 MINUTES LATER 3 OF 4 FLIGHT ATTENDANT AND SOME PASSENGERS COMPLAINED OF SMELLING FUMES AND ASLIGHT HEADACHE. IT WAS MOSTLY AT THE REAR OF THE CABIN. REPLACED O-RING "SEAL" (AALA202503252772) BETWEEN APU "B" FCU AND OIL LUBE UNIT, PER AMM 49-31-11. REMOVE AND REPLACED ELEVATOR A/P "B" ACTUATOR (AALA202504014312), PER AMM 22-11-26. CLEANED APU COMPRTMENT, PER AMM 49-16-11. ACCOMPLISHED SOF TROUBLESHOOTING J/C #08L00021001. NO ODOR ADDTIONAL ENCOUNTERED.