As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
UPON LIFTOFF FUSELAGE DOORS WARNING WAS INDICATED WITH THE AFT CARGO DOOR SHOWING OPEN. CABIN WAS NOT PRESSURIZING. RETURNED TO FIELD LANDING UNEVENTFUL. REMOVED FROM SERVICE. C/W OPS TEST OF AFT CARGO DOOR PER Q400 AMM 52-31-00-710-801 AND INSP PER Q400 AMM 05-52-00-000-000. NO DEFECTS NOTED.