As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON GROUND @ IDLE LEFT DUCT PRESSURE IS LOW AND FLUCTUATING BETWEEN 7 & 12 PSI. GROUNDED - YES. REMOVED & REPLACED HIGH STAGE VALVE PER B737-800 AMM 36-11-06 PB 401. OPS AND LEAK CHECK GOOD. REMOVED AND REPLACED HIGH STAGE REGULATOR PER AMM 36-11-07 PB 401. OPS & LEAK CHECK GOOD, NO FAULTS FOUND.