As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER DEPARTURE DURING CLIMB EICAS MESSAGE L PACK OFF AND L PACK TEMP LIGHT CAME ON. FOLLOWED QRH WITH EVENTFUL LANDING BACK AT JFK. TROUBLE SHOT IN ACCORDANCE WITH AMM 36-11-00. REPLACED 490 AIR SUPPLY THERMAL OVER TEMPERATURE SWITCH, OPS NORMAL.