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

Attributes
ACN	1621776
Time
Date	201902
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Climb
Flight Plan	IFR
Component
Aircraft Component	Pressurization Control System
Person 1
Function	Pilot Flying First Officer
Qualification	Flight Crew Multiengine Flight Crew Instrument Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 200 Flight Crew Total 18000 Flight Crew Type 5000
Person 2
Function	Pilot Not Flying Captain
Qualification	Flight Crew Instrument Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Critical

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.

Original NASA ASRS Text

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

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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.