Maintenance Powerplant; Maintenance Airframe
Aircraft Equipment Problem Less Severe
Air Traffic Control Provided Assistance; Flight Crew FLC Overrode Automation; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Departure Airport; General Maintenance Action
Aircraft
The aircraft returned to ZZZ for a loss of Cabin Pressurization. Altitude Alert sounded to notify crew of an emergency. Climbing through FL230; lost the cabin pressurization. Altitude Alert sounded. Captain advised First Officer (F/O) to run the check list; Captain had aircraft and radios. Requested immediate descent to MSA. Was given confirmation to descend to 14;000 ft; then continue descent to 10;000 ft. F/O and myself determined that we did not have Auto and STBY modes for cabin pressurization. For the safety of the crew and passengers; we requested a priority landing ; because I was not sure if I could maintain cabin pressurization in the Manual Mode. We returned to ZZZ; I was advised to do an operational test of the outflow valve in the manual mode by Maintenance Control. I performed a ops test of the outflow valve in the manual mode; and the ops test passed. Maintenance Control advised me that I was to MEL and DMI the cabin pressurization and dispatch the aircraft. I dispatched the aircraft under MEL.
Maintenance Technician reported the pressurization system on a B737 was inoperative in auto and standby modes.