As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
LEFT DUCT PRESSURE FLUCTUATING FROM 25 - 45 PSI AT 41000 FEET CRUISE ALTITUDE. REPLACED LT ENGINE PRSOV REGULATOR IAW AMM. CHECK PNEU BLEED LINE TO PRSOV AND REV FLOW VALVE AND PRSOV REG ALL LINE ARE SECURED AND TIGHT. ACP'D ENGINE RUN WAS GOOD LT AND RT PNEU BLEED PSI WERE MATCHED WITH NO FLUCTUATIONS.