Date: 202201
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: B737-700
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: System Monitor: Indicating and Warning
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; First Officer
Reporter qualification: Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Human factors: Troubleshooting
Second reporter function: Captain; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Gate; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Rejected Takeoff; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: When the Captain advanced the throttle levers prior to the Before Takeoff Checklist; the takeoff warning horn sounded and the associated warning light illuminated. We stopped the aircraft and began looking for any configuration error that could be causing the light to go off. We looked all over the flight deck; including the overhead panel; and could find nothing out of place or anything that would trigger this horn and light. It was a flaps 1 takeoff and they were at 1 and showed 1. The trim was properly set at between 5 and 6 to my recollection. The speed brake was down and stowed; and we checked it; jiggled it; it was definitely where it was supposed to be. All the door lights were out as well as the PSEU (Proximity Switch Electronics Unit) light.I had flown the airplane once that day and had no issues at all; so we put the flaps back up; waited a beat; and then reset the flaps to 1. The Captain then performed another throttle burst. We could not get the warning horn or the light to go off this time. We re-checked the configuration and probably did two handfuls of throttle bursts to make sure; and we could not get the issue to repeat itself. The airplane appeared fine. We continued our taxi and received our lineup and wait clearance and eventual take off clearance. It was the Captain's takeoff and the takeoff roll was normal. V1 was 132 knots. At 125 knots I was opening my mouth to call V1 as the aircraft continued accelerating; and at that very instant; the warning horn and light went off. The Captain performed the RTO per the QRH. After completing this; we then pulled the brake cooling data from the performance system; and it said we could return to the gate and required 50 minutes of brake cooling. We gave all the information to Maintenance and called Dispatch and Maintenance Control.


Second narrative: Our takeoff warning horn activated during the throttle check portion of the Before Takeoff Checklist. I stopped the aircraft; and we checked for all the items that would activate the takeoff warning horn. We did not find anything that was out of position to trigger the takeoff warning horn-flaps were at 1; trim in the green band; speed brake lever handle down and stowed; all door lights were out; and no PSEU light. As we troubleshot the takeoff warning horn; we made sure the speed brake handle was down and stowed; and we cycled the flaps from Flaps 1 to UP and back down to Flaps 1. After doing that; the takeoff warning horn could not be activated again. I must have tested the throttles and tried to active the takeoff warning horn 8-10 times before taking the aircraft onto the runway for takeoff. Both the First Officer and I agreed that the takeoff warning horn problem was resolved before we proceeded to taxi again to the runway for takeoff. Auto throttle was engaged for our takeoff. V1 was at 132 knots. At 125 knots we got the takeoff warning horn and the takeoff configuration warning light. I felt the aircraft fell into the unsafe category at that time and announced 'reject' and performed the Rejected Takeoff procedure per the QRH. After running the QRH; we calculated the RTO brake cooling requirement to be 50 minutes at the gate. We returned to the gate and gave all the information we had to Maintenance.


Synopsis: B737-700 flight crew reported a rejected take off after the T/O Configuration Warning Light and T/O Warning Horn sounded at 80 knots.