As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AS FLIGHT 4169 DEPARTED THE CREW SMELLED AN UNUSUAL ODOR IN THE COCKPIT AND BECAME LIGHT HEADED. THE CREW IMMEDIATELY RETURNED TO THE FIELD AND PROCEEDED TO THE GATE. MAINTENANCE INSPECTED THE AIRCRAFT AND DEFERRED THE AUXILARY POWER UNIT, FLIGHT 4169 THEN DEPARTED WITHOUT FURTHER INCIDENT.