Date: 202104
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Light: Daylight
Aircraft: B737 Next Generation Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Pressurization Outflow Valve
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Human factors: Workload; Distraction; Troubleshooting
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Overcame Equipment Problem; Flight Crew Diverted; Flight Crew Landed As Precaution; Flight Crew Landed in Emergency Condition; General Maintenance Action; General Flight Cancelled / Delayed
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Flight was scheduled to depart ZZZ. We arrived in the flight deck and performed all the normal procedures we have for a normal push back. Captain did the preflight walk around and I loaded the FMC. We agreed the OFP (Operational Flight Plan) looked good and the rest of the flight plan looked sufficient for a potentially bumpy ride to ZZZ1. Captain advised to Flight Attendants to stay seated out of ZZZ until we knew the ride would allow for their safe duties. I was the flying pilot and after Captain did his first flight of the day and PM (Pilot Monitoring) duty brief; I briefed the SID. We planned for a normal pushback with 2 engine start for a quick taxi for departure. We performed the BEFORE TAKEOFF CHECKLIST on taxi out down the ramp for takeoff on runway XX.While we were taxiing down to runway XX; I noticed there was a split between the L and R duct pressure. I made note of it to Captain and we briefly discussed these splits we have seen before in this aircraft and recall it being a known normal occurrence. I cycled the L PACK (Left) switch to OFF and back to AUTO as well as the ISOLATION VALVE switch to OPEN and AUTO and saw the duct pressure needles sync together. We decided it would be a safe action to depart and noted we would keep an eye on the duct pressure and pressurization.We departed runway XX and CLIMBED VIA THE SID. I hand flew the departure to the mid 20s and then turned the autopilot on. I noted Captain mentioned 2 different times that the aircraft was pressurizing normally though the duct pressure still was split. The ride was mostly good and the flight attendants were up but the seat belt was still on. As we approached our level off altitude of FL340 the CABIN ALTITUDE horn and the MASTER WARNING lights illuminated. We verified the CABIN ALTITUDE was high on the pressurization panel and performed the Immediate Action items for CABIN ALTITUDE WARNING OR RAPID DEPRESSURIZATION/EMERGENCY DESCENT. The CABIN ALTITUDE was not controllable; and we continued with the descent. There was a definite issue with immediate communication over the inter-phone with the masks on and all the noise. Getting the mask on and the audio panel correct to be able to speak and hear over the speaker poses a threat in this airplane if not practiced. We were able to communicate with each other and ATC but it was challenging with all the noise and distractions.Captain identified as a priority flight with ZZZ Center and we began the initial descent over the mountains. I initially turned off the AUTOPILOT to pitch over and begin the procedure. Once we were established in a descent; I turned the AUTOPILOT back on and descended to FL190 with the high terrain in mind. We saw an aircraft below us on the TCAS and had to shallow the descent initially before we were cleared lower. Captain proceeded with completing the checklist and communicating with the Flight attendants; passengers; and dispatch. Descending through approximately 16;000 feet I noticed the CABIN ALTITUDE was in a normal range of around 7;000 feet and we took off the oxygen masks. We leveled off at 11000 in the vicinity of ZZZ2. Initially we were planning on diverting to ZZZ2 because we were so close; but Captain's experience and familiarity with ZZZ3. This led to the decision to divert and there was difficulty communicating with dispatch possibly due to our low altitude so we opted to landing in ZZZ3. The flight attendants and passengers were all briefed for the diversion to ZZZ3. I left the AUTOPILOT on and we flew the RNAV (GPS) Runway XY 29 approach and landing. All checklists were completed before ZZZZZ intersection and the landing was normal. CFR ( Aircraft Rescue & Firefighting) was there upon arrival.We were parked at the hard stand in ZZZ3 and the ramp personnel brought over the ramp to allow the passengers to vacate the aircraft. Operations were there to help while we figured out the plan of action for the passengers. Captain discussed the situation with the ChiefPilot; Dispatch; and Maintenance and I aided the flight attendants with passengers. I walked to the OUTFLOW VALVE and noted a mesh substance blocking the OUTFLOW VALVE from fully closing. Captain manually opened and closed the valve and I videoed the movement.




Synopsis: First Officer reported an emergency descent and diversion followed by a precautionary landing caused by Foreign Object Debris blocking the outflow valve from operation.