As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
RETURNED ORD DUE TO AIRCRAFT WOULD NOT MAINTAIN CABIN PRESSURIZATION. RIGHT PACK ON MEL. MAINTENANCE FOUND GROUND AIR CHECK VALVE OBSTRUCTED. RESEATED CLAMPS ON CHECK VALVE. REPLACED DAMAGED PLENUM AND SECONDARY HEAT EXCHANGER, REPLACED 4 MISSING FUSELAGE DRAINS AND RIGHT FLOW CONTROL VALVE. REFER TO B737 AMM 21-51 AND AMM 51-41. (LOG 7604038 - AIRCRAFT WAS GROUNDED).