Air carrier flight crew reported receiving a warning message during departure that a door was open; although it was reportedly locked and closed. The flight crew returned to departure airport safely. Maintenance cleared the flight to depart and the same message appeared during descent to next airport.
Synopsis
Air carrier flight crew reported receiving a warning message during departure that a door was open; although it was reportedly locked and closed. The flight crew returned to departure airport safely. Maintenance cleared the flight to depart and the same message appeared during descent to next airport.
Narrative
While flying the ZZZZZ departure out of ZZZ for ZZZ1 after passing 15;000 ft. the Master Warning chimed and an EICAS Warning message of 'DOOR PAX AFT OPEN' appeared. I; the First Officer; was flying the aircraft with the autopilot on at the time; the Captain was pilot monitoring. When the chime went off I called to identify and cancel the message; the Captain read the EICAS Warning message of 'DOOR PAX AFT OPEN'; we then asked ATC to stop our climb and leveled off at 15;500 ft. At the same time the Captain pulled out the QRH and pulled up the appropriate checklist. At this time we also called the flight attendants to verify that the L2 door was in fact not open; the AFT FA visually checked the door and stated that the door was closed and locked. At this time we continued to run the QRH and requested a descent after noticing that the cabin altitude seemed to be slightly higher then what seemed normal for that altitude. While discussing the issue add referencing the checklist we determined that the best course of action was to [request priority handling] and return to ZZZ. While coordinating a return to ZZZ with ZZZ Departure we reached out to dispatch to inform them of our issue. Dispatch came back and recommended that we got to ZZZ2 if we were having a maintenance issue. We looked up our current distance to ZZZ2 and ZZZ and it would have taken three times longer to go to ZZZ2 then to return to ZZZ; so after discussing the issue we decided that the nearest suitable airport to return to was ZZZ. We informed ATC that we wanted to continue our return to ZZZ and requested a lower altitude per the QRH. Part of our decision to return to ZZZ was that the issue originally came on 15;000 ft; then went away at 13;000 ft; then came back on again 11;500 ft and once again extinguished at 9;500 ft. We were unsure what may be the problem and felt the safest course of action for the crew and passengers was to return to ZZZ. The Captain using the EMER Call Button called the FA's; let them know of the situation; and came up with a plan of action which involved a normal landing back at ZZZ. The rest of the flight was uneventful and we landed back at ZZZ; returned to a gate; and called maintenance. Maintenance arrived checked out the door and deferred the door using MEL XX-YY-YY-Z and returned the aircraft to service. Upon climbing out of ZZZ the second time we had the same issue appear at 15;000 ft; per the MEL we contacted the FA's to ensure the door was indeed still closed and latched and continued on to ZZZ1 with the EICAS Warning message per the MEL. On descent into ZZZ1 the message once again went away. Earlier in the day maintenance had stated they fixed the L2 door so on our first flight out of ZZZ we had no active MEL's. If maintenance had not cleared the open write up for the L2 door earlier in the day we would have referenced MEL XX-YY-YY-Z prior to the first flight to ZZZ1. If the write up had still been active we would have been able to see that the message may appear in flight and would have continued on to ZZZ1. Because their was no active maintenance issues with the aircraft prior to our first attempt to ZZZ1 we decided that the safest course of action was to return to the field.
Second reporter narrative
While climbing out through 15000. EICAS message Pax Service AFt Door open (Red). Appeared. We leveled out around 16000 and start doing the QRH procedure. Went to QRH steps; spoke back to the FA's. Everything was showing green indicators for L2 door. Pressurization was around 4000 Cabin Alt; kinda high for 15000. But Green. Me and the FO decided given a 2.15 Hr flight ahead of us and not sure what exactly the reason behind this message; as precaution we will go back to ZZZ. [Advised ATC] and chose to go back to ZZZ. Even though Dispatch told us to go to ZZZ2 which was 1XX miles from our position. While descending thru 13000 feet the message went away. At 11500; the message warning came back on again. At 9000 the message went away. No overweight landing. Taxied to the gate. Maintenance and Dispatch was notified simultaneously. Spoke to the Duty Officer. Get the new release and Maintenance wrote up as a new MEL. For the L2 door Latch Indicators. As Precaution we [requested priority handling] and came back to the field. After getting the new release and taking off again for ZZZ1; at 15000 feet the same message comes back again but this time the Cabin Alt was at 2500 instead of 4000. Dispatch and Maintenance was notified to keep them in the loop since couldn't reciprocate the problem on the ground after coming back to ZZZ.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.