As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER PUSHBACK, DURING TRANSITION FROM START/FEATHER TO MAXIMUM WITH THE CONDITION LEVERS, AIRCREW NOTICED A BURNED ODOR COMING THROUGH THE VENTS IN THE FLIGHT COMPARTMENT. AIRCREW RETURNED TO GATE, GROUND MAINTENANCE CREW REMOVED AND REPLACED THE AIR CYCLE MACHINE. OPS CHECKS WERE PERFORMED, SYSTEM CHECKED SATISFACTORY. AIRCRAFT RETURNED TO FLIGHT STATUS.