As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
GATE RETURN FOR UNUSUAL ODOR. ODOR SLIGHTLY PRESENT WITH APU AND MUCH MORE PRONOUNCED WITH ENGINE 2. PERFORMED SMOKE/ODOR IN FLIGHT DECK/CABIN CONDITIONAL INSPECTION WITH APU AND ENGINES RUNNING IAW AMM 05-51-26-800-801-C1. NO ODORS NOTED.