What was found
ON TAKEOFF ROLL LEFT PACK LIGHT ILLUMINATED AFTER TAKE OFF. WE RAN THE PACK LIGHT CHECKLIST IN THE QRH. THE LIGHT EXTINGUISHED AND NEVER REILLUMINATED. GROUNDED - YES. FOUND LEFT PACK EXHAUST PLENUM FAN BYPASS CHECK VALVE MISSING. REPLACED LEFT PACK PRIMARY DIFFUSER DUCT REF AMM 21-51-03. NR 1 PACK ZONE TEMP CONTROLLER BITE COMPLETED REF AMM 21-61-00. NO FAULTS FOUND OPS CKS GOOD. PERFORMED CABIN TEMP CONTROL AND INDICATION OPERATIONAL TEST REF AMM 21-61-00. NO FAULTS FOUND.