As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CABIN PRESSURIZATION FAULT LIGHT ILLUMINATED INTERMITTENTLY. DURING MEL MAINTENANCE PROCEDURE(21-30-5(A)) STEP 12 FWD OUTFLOW VALVE FAILED TO DEPRESSURIZED AIRCRAFT. REPLACED CABIN PRESSURE CONTROLLER IAW AMM TASK 21-31-06-000/400-801, FAULT DID NOT CLEAR IN CDS. R AND R AFT OUTFLOW VALVE IAW Q400 AMM TASK 21-31-16-000/400-801 FUNCTIONAL TEST OF PRESSURIZATION CONTROL PASSED. FAULT CLEARED. RETURNED TO SERVICE.