As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: SHORTLY AFTER TAKE OFF, BURNING TYPE ODOR WAS NOTICED IN COCKPIT AND THROUGHOUT CABIN. COULD NOT IDENTIFY SOURCE. AIR-RETURN FOR PRECAUTINARY LANDING. AGENT MEETING FLIGHT NOTICED ODOR UPON OPENING 1L DOOR. MAINTENANCE ACCOMPLISHED CARD 05-96-70 AND FOUND THE APU COMPARTMENT OIL SOAKED AND DECLARED AN APU CHANGE. REMOVED AND REPLACED THE APU IAW CARD 49-96-83. OPS CHECK AND LEAK CHECK GOOD.