Date: 201902
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B737 Undifferentiated or Other Model
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 Control System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; First Officer
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Second reporter function: Pilot Not Flying; Captain
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Took Evasive Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Climbing through FL 210 we got a cabin altitude warning and horn and passenger oxygen masks deployment. We immediately donned our oxygen masks and established communication. We then notified ATC of our problem and leveled off at FL 220. The captain ran the QRC and QRH while I flew the aircraft. We noticed the cabin altitude was stable at 15;000 feet; although this was neither an explosive or rapid decompression we clearly had a pressurization problem with only partial pressurization; so we [advised ATC] and descended to 10;000 feet. We then requested a turn back to ZZZ and finished our checklist and requested overweight landing data as we would be 800 pounds over max landing weight. We did a normal flaps 30 landing followed by normal brake application. After turning off the runway we had the [ARFF personnel] do an inspection of our landing gear for any hot brakes. After getting the all clear we proceeded to the gate and deplaned. The maintenance technician then advised us of a failed one way check valve in the external ground air receptacle which was the cause.


Second narrative: Climbing through flight level 210 we got the cabin altitude warning horn. We stopped the climb at flight level 220. First Officer (FO) continue flying the aircraft and I ran the QRC. I [advised ATC and the FO] started the descent for 10;000 feet. At 22;000 feet we notice the cabin was holding around 15;000 feet. I informed the flight attendants about our situation and that the masks where deployed. I informed ATC that we were going back to ZZZ and continued our descent to 9;000 feet. We finished the QRH and called FA that it was safe to take oxygen masks off. I made a public address to the passenger informing them of what just happened; and that we are returning to the field. I tried to call ZZZ operations to tell them that we were coming back; but no luck. ZZZ1 operations was able to relay to ZZZ that we were going back. I sent a message to dispatch informing them; that we were returning to ZZZ and for station to be ready for us. I asked for landing data; it looked like we were going to be within 1;000 lbs. max structural LDG weight. We reviewed the overweight landing section. I asked to have [the] equipment standing by; FO continued flying the aircraft. Normal approach and landing; pulled off the runway; had gear inspected by the ground personnel. I informed the passengers of what was going on; and about the   equipment around the aircraft. Once they cleared us; I taxied the aircraft to the gate. All passengers seemed to be in good spirit. A mechanic came up to cockpit and showed us a pin he found in the external ground air valve port. He said; that it holds the check valve in place; and it was off the hinge. I appreciate the professionalism of my crew and how well they performed under extraordinary pressure. My FO as well as the flight attendants all did their job outstandingly. The flight attendants walked the whole cabin and made sure every passenger had their oxygen masked on correctly.


Synopsis: B737 flight crew reported a pressurization issue led to a return to the departure airport.