Date: 201912
Local time of day: 0601-1200
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Light: Night
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Pneumatic Valve/Bleed Valve
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Second reporter function: Pilot Not Flying
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Maintenance; Flight Deck / Cabin / Aircraft Event Other / Unknown
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed in Emergency Condition; Flight Crew Diverted; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft; Procedure
Primary problem: Aircraft

Narrative: The aircraft was indicated to have arrived in ZZZ from ZZZ1. The inbound crew recorded an aircraft discrepancy in the AML for a 'BLEED TRIP OFF' on final approach into ZZZ; which the prior crew noted was successfully reset using QRH procedures.At the direction of Company Maintenance; an engine run was performed in ZZZ in accordance with Company Maintenance procedures to evaluate the status of previously noted BLEED TRIP OFF write up prior to logbook sign off. A Contract Maintenance Technician overseen by Company Maintenance provided test card procedure; accompanied the two pilots for an off-gate medium/high power engine run. The results of the engine run procedure indicated normal pneumatic system operation with no defects noted.After the engine run; the aircraft returned to the gate so that bags could be loaded and passengers boarded. At XA33; the aircraft departed ZZZ enroute for ZZZ1. Normal systems operations were noted for taxi; takeoff; climb - with intermittent level-offs - and approximately 35-minutes of cruise flight at FL380. A very slight pressure bump was felt which coincided with the R BLEED TRIP OFF light illuminating on the Pneumatics System overhead panel. After identifying the Pilot Flying; the QRH was referenced for the appropriate checklist procedure. The BLEED TRIP OFF light failed to reset; which resulted in the R A/C Pack being placed in the 'OFF' position in compliance with QRH procedure.Shortly after the completion of the QRH procedure; the crew discussed and agreed that a descent from FL380 to a lower altitude and lower PSID would probably be a wise course of action. A lower altitude was requested and ATC began to coordinate and approve the aircraft descent. Upon passing approximately FL360; the cabin altitude was noted to be rising and the red CABIN ALTITUDE Warning light illuminated; accompanied by the warning horn. The QRC and QRH were referenced for the appropriate procedures. With the gradual loss of cabin pressurization; not immediately arrested by procedure; the crew complied with emergency descent procedures and [requested priority handling] with ATC. The cabin altitude continued to climb; which was noted to top out at nearly 12;000 feet on the pressurization dial; while the emergency descent procedure was maintained.The aircraft was leveled at 10;000 MSL; and the cabin was still partially pressurized. A check with the cabin crew confirmed that the passenger oxygen masks had not deployed. The flight diverted to ZZZ2 where an uneventful approach and landing were executed. All passengers and crew deplaned normally and without any noted issues.


Second narrative: [Report narrative contained no additional information.]


Synopsis: B737 flight crew reported a recurrence of a BLEED TRIP OFF light inflight; causing a loss of cabin pressure; and resulting in a diversion and precautionary emergency landing.