As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING TAXI WHILE AFTER STARTING L ENG MASTER WARN AND MAIN CARGO DOOR WARNING LIGHT CAME ON AC RETURNED TO GATE AND MTX GROUNDED AC . MTX PERFORMED MAIN DECK CARGO DOOR SYSTEM TEST PERFORMED REF AMM 52-32-00 AND SAMM 52-71-09 OP CHECK GOOD. MTX RETURNED AC TO SERVICE AND DEPARTED