As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER ENGINE STARTS FLIGHT ATTENDANTS REPORTED BURNING PLASTIC ODOR AS WELL AS STRONG GAS/FUEL SMELL. BOTH FIRST OFFICER AND I ALSO SMELLED THE SAME ODORS. TURN OFF PACKS AND RETURNED TO GATE. FOUND SOURCE OF CONTAMINATION FROM AIR START CART. SEE LOG PAGE 1839883. (LP: 9153420, AIRCRAFT WAS GROUNDED)