As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AT CRUISE FLIGHT FL 340 THE LEFT DUCT PRESSURE CYCLES BETWEEN 30PSI AND 40PSI CONTINUOUSLY. THE CABIN RATE OF CLIMB VARIES BETWEEN +500 AND -500 FT/MINUTE AT THE SAME TIME THE PRESSURIZATION REMAINS CONSTANT AT 7500 CABIN ALT AND 7.8 PSI. GROUNDED - YES. REMOVED AND REPLACED LEFT PACK FLOW SENSOR REF B737-900 AMM 21-51-43 OP CK GOOD. PRESSURIZED AIRCRAFT USING BOTH PACKS TOGETHER AND SEPARATELY WITH NO FLUCTUATION OF DUCT PRES OR RATE OF CLIMB.