As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT SLOWLY LOST PRESSURIZATION AT 300 FPM DURING CRUISE FLIGHT. TROUBLESHOT THE PROBLEM USING THE BOEING 737 FIM. DETERMINED TO BE LOW AIR DUCT PRESSURE COMING FROM THE RIGHT ENGINE. USING STEPS LISTED IN THE FIM TO TEST VALVES ON THE RIGHT ENGINE, DETERMINED THE RIGHT PRESSURE REGULATING AND SHUTOFF VALVE NEEDED TO BE CHANGED. CHANGED THE VALVE AND PERFORMED ENGINE RUNS. RIGHT ENGINE AIR DUCT PRESSURE WAS NOW CORRECT.