As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
LEFT CABIN DOOR SHOWED UNLOCKED AT FL 240. LEVELED OFF, PERFORMED QRH & PUSHED ON THE HANDLE. DOOR SHOWED LOCKED. CONTINUED TO CLIMB & DOOR SHOWED UNLOCKED AT FL 340, FINAL ALTITUDE WAS TO BE FL 400, RETURNED TO DEPARTURE. REMOVED AND REPLACED DOOR WARNING SWITCHES 7WS AND 43WS IAW AMM 52-71-11-04 AND TESTED IAW AMM 52-71-11-05 AND 52-71-00-05. DOOR WARNING OPERATES NORMAL AND ACFT IS OK FOR SERVICE.