As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AT CRZ FL 370, DUCT PRESS L+R FLUCTUATING ALTERNATIVELY BETWEEN 30-40 PSI AT SAME TIME, CABIN ALT RATE CLIMBING 0.5-1.0 AND DESCENDING 0.5-1.0 20-30 SECONDS APART. GROUNDED - YES. AIR TURNBACK-DIVERSION TIME 115 MINUTES. REMOVED AND REPLACED L/H FLOW CONTROL VALVE REF B737-900 AMM 21-51-01-400-807-003. OPS CHECK GOOD. NO LEAKS NOTED ON DUCT JOINTS ON NO.1 HIGH PRESSURE WATER SEPARATOR REHEATER AND CONDENSOR. REPLACED PACK FLOW SENSOR #1 L/H SIDE REF AMM 21-51-43. PFTC BITE TEST GOOD. REF AMM 21-61-00-740-801-002.