Date: 202201
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Takeoff / Launch
Airspace: Class B ZZZ
ATC / advisory: Tower ZZZ
Aircraft component: APU Fire Extinguishing
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Troubleshooting
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Landed in Emergency Condition; Flight Crew Overcame Equipment Problem; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Departure Airport; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Aircraft just came out of a C check; on takeoff roll received master caution OVHT/DET around 100 kts; on [takeoff] rotation Fire Warning annunciated along with bell and APU Fire pull handle illuminated. These indications flickered on and off and then off. Requested level off to work problem and to stay close to the field. Warning did not come back. From what we saw we believed the problem was just an indication problem that went away. We elected to proceed to ZZZ1. Then almost immediately the Fire warning came back and stayed continuously illuminated with bell only flickering off a few times. Did immediate action items; pulled APU Fire switch and fired the bottle. Requested priority handling. Continued with QRC and Fire illumination did not go away for good so per the QRC we landed at nearest suitable airport; ZZZ. Before landing I notified our FAs (Flight Attendants); notified our Passengers and [contacted] Dispatch. After landing had Fire Rescue trucks inspect aircraft. Once given all clear; we taxied to the gate.


Second narrative: During takeoff roll; we received an OVHT/DET light on the Captain's 6 pack at approximately 100 kts. The Captain chose to continue the takeoff. Once airborne and safely climbing away from the ground; I noticed the APU DET INOP light was illuminated. The APU was not running and was successfully shutdown during normal Ground Operations before departure. As we began to run the QRH; we received intermittent FIRE warnings for the APU. The FIRE warnings lasted just a few seconds and were intermittent. We decided to return to ZZZ. As we completed the QRH for the APU DET INOP; the Intermittent FIRE light stopped completely. After discussing with the Captain; we felt comfortable continuing to ZZZ1. As we began a climb back to the north towards ZZZ; the FIRE light came back on again; and again was intermittent but would stay on for no more than 5 seconds; go off for 1-2 seconds; then back on again for a few seconds. The FIRE light was also illuminating intermittently in the APU FIRE handle. At this point; we decided to return to ZZZ. The Captain performed the Immediate Action items while I flew the plane and talked to ATC. He then ran the QRC and the QRH; while I followed along and got the jet ready for the return (programmed the box; sent the divert message thru ACARS; etc.) After running the non-normal checklist; the Captain flew the plane and I ran the normal checklists. After landing; ARFF inspected the aircraft to include the APU area and found nothing abnormal. They followed us to the gate and the event was terminated.


Synopsis: B737 flight crew reported intermittent problems with the APU Overheat/Detection and Fire Warning systems. An air turn back and precautionary landing were made at the departure airport.