As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) ON TAXI OUT, A DEADHEADING ACFT EMPLOYEE NOTICED A BURNING ODOR IN THE AFT CABIN, THE F/A AGREED THAT WAS A STRONG BURNING RUBBER ODOR. THE CREW ELECTED TO CALL THE FIRE TRUCKS OUT AND RETURN TO THE GATE. NO SMOKE ODOR EVIDENT ON RTG. BOTH ENGINES RUN FOR T/S. NO ABNORMAL ODORS EVIDENT ON PACK RUNS AFTER ENGINE STARTS.