As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WITH BOTH ENGINES RUNNING, POWER AT IDLE, BOTH PACKS IN AUTO, LEFT DUCT PRESSURE READS 10 PSI. ENGINE ANTI ICE OFF AS WELL. GROUNDED - YES. PERFORMED DUCT PRESSURE LOW FAULT ISOLATION REF B737-900 FIM 36-10 TASK 804. REMOVED AND REPLACED BLEED AIR REGULATOR REF B737-900 AMM 36-11-03/401. PERFORMED ENGINE RUN REF POWER ASSURANCE CHECK REF AMM 71-00-00-700. OPS CHECK WAS NORMAL.