Commercial Fixed Wing
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Diverted; General Maintenance Action; General Flight Cancelled / Delayed
Aircraft
At FL210 master warning fuselage doors illuminated. Ran appropriate checklists; and talked to the flight attendants. It was deemed that the aft passenger door was not properly seated. We advised ATC and decided to divert to the nearest suitable airport (ZZZ). Safe outcome of flight.We eventually got on call maintenance to the aircraft; without having a definitive answer; it was decided the most likely issue was that the door may have never been closed and latched properly. The aft passenger door was opened and closed in ZZZ1.After maintenance ops check good; and getting refueled we continued to ZZZ2.I do feel as though our crew handled the situation well. I think some take aways are that while we were closest to ZZZ; there was a thought in the back of my head thinking that turning back to ZZZ1; or continuing to ZZZ3 were also good options; however the checklists drove us to nearest suitable which was ZZZ. In the future I would probably make the same decision; but it has given me thought. I also wonder if it would be good procedure to have the flight attendants just push on the door handle prior to departure to have one more check on the door security. One of our flight attendants had this exact same experience on this airplane approximately 1 week prior so there seems to be a systemic issue to address here.
Captain reported a diversion after the aft entry door triggered the Master Warning Doors Light.