As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DIVERTED TO ATL DUE TO PRESSURIZATION PROBLEM & CABIN ALTITUDE WARNING. FOUND UPPER POSITIVE PRESSURE RELIEF VALVE OPEN WITH INSULATION STUCK IN VALVE. REPLACED POSITIVE PRESSURE RELIEF VALVE AND REMOVED LOOSE INSULATION AROUND VALVE IAW AMM. PERFORMED CABIN HULL LEAK CHECK, SINGLE PACK CONFIDENCE CHECK, AND OUTFLOW VALVE TESTS.