As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER BOTH ENGINES STARTED L PACK LIGHT CAME ON. L PACK LIGHT WOULD NOT GO OUT ON RECALL, DID NOT GO OUT ON TRIP RESET. RAN QRH AND RETURNED TO THE GATE. AFTER RETURN TO THE GATE ENGINES OFF PACK LIGHT WENT OFF. GROUNDED - YES. R & R L PACK TEMPERATURE CONTROL VALVE REF B737-800 AMM 21-51-10/401. OPS CHECK AND LEAK CHECK GOOD.