As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CONSTANT 1500 FPM DESCENT, NO CHANGE TO POWER LEVERS OR INPUT TO PRESSURIZATION CONTROLLER, CABIN WENT FROM CONSTANT 500 FPM DESCENT TO A CLIMB RATE OF 3000 FPM, THEN A DESCENT RATE OF 3000 FPM, THEN STABILIZED. CABIN DIFFERENTIAL APPROX 1.5 PSI AT THE TIME. ACFT ALTITUDE APPROX 9000 FEET. MX R & R CABIN PRESSURE CONTROLLER IAW Q400 AMM TASK 21-31-06-000-801. COMPLIED WITH FUNCTIONAL CHECK OF PRESSURE CONTROL. OPS CHECK GOOD. ACFT RETURNED TO SERVICE.