Narrative:

We initiated takeoff roll runway 12R. Captain flying. Flaps 1. Reduced power with damp runway. Takeoff progressed normally. At about 120 knots; the red takeoff configuration light illuminated accompanied by the intermittent aural alarm horn. I immediately was concerned that an unknown configuration change had occurred that would make the aircraft not airworthy; so I initiated a rejected takeoff. The aircraft deceleration with rejected takeoff (rejected takeoff) autobrakes engaged was smooth and progressive to full stop about 6;000 feet down the 11;500-foot runway. I made the repetitive 'remain seated' PA to the cabin. The first officer (first officer) informed tower we had stopped the takeoff for a configuration warning and were assessing the situation for a moment. Within a minute or so; I concluded that nothing had happened that would preclude safe aircraft movement; so we told tower we could clear the runway and proceed to a gate. We taxied to assigned hardstand; left the flaps at 1; ran the after landing and parking checklists; wrote up the event in the logbook; and met maintenance personnel who began investigating the nature of the problem.after parking at the hardstand; we discussed with maintenance personnel what happened. They first confirmed that all flaps were in the appropriate position for flaps 1. They began diagnosing the problem; coordinating with [the company]; and running through diagnostic checklists. Within about 30 minutes; they felt the problem had been isolated to the #3 leading-edge flap on the left wing which had apparently caused the recording of a nonspecific 'leading edge not extended' entry in the bite (built-in test equipment) computer during takeoff roll. They removed an access panel under the leading edge of the left wing; found some paint chips between the 'sensor' and the 'target' that they believed would have caused erroneous report. After clearing the debris; they ran the flaps through all settings several times without any indication of defect; closed the panel up; signed us off; and away we went.the maintenance personnel were knowledgeable about what to check and how to check it and wasted no time in getting to the problem and fixing it. Shortly after parking; I suspected a sensor problem was the cause (not actual flap malfunction); so I told the station that we would not plan on deplaning right then. The passengers were very cooperative and understanding. As the investigation and repairs progressed; I made at least four rather long pas in which I relayed the known issues at the time and what next steps might be coming. Probably nothing could be done to avoid this. Takeoff configuration is not an alarm that I ever expected to get at high speed but I believe I made the right call to reject given the circumstances.

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Original NASA ASRS Text

Title: B737-800 Captain reported rejecting takeoff at about 120 kts when a TAKEOFF CONFIG light illuminated.

Narrative: We initiated takeoff roll RWY 12R. Captain flying. Flaps 1. Reduced power with damp runway. Takeoff progressed normally. At about 120 knots; the red TAKEOFF CONFIG light illuminated accompanied by the intermittent aural alarm horn. I immediately was concerned that an unknown configuration change had occurred that would make the aircraft not airworthy; so I initiated a rejected takeoff. The aircraft deceleration with RTO (Rejected Takeoff) autobrakes engaged was smooth and progressive to full stop about 6;000 feet down the 11;500-foot runway. I made the repetitive 'remain seated' PA to the cabin. The FO (First Officer) informed tower we had stopped the takeoff for a configuration warning and were assessing the situation for a moment. Within a minute or so; I concluded that nothing had happened that would preclude safe aircraft movement; so we told tower we could clear the runway and proceed to a gate. We taxied to assigned hardstand; left the flaps at 1; ran the after landing and parking checklists; wrote up the event in the logbook; and met maintenance personnel who began investigating the nature of the problem.After parking at the hardstand; we discussed with maintenance personnel what happened. They first confirmed that all flaps were in the appropriate position for Flaps 1. They began diagnosing the problem; coordinating with [the company]; and running through diagnostic checklists. Within about 30 minutes; they felt the problem had been isolated to the #3 leading-edge flap on the left wing which had apparently caused the recording of a nonspecific 'leading edge not extended' entry in the BITE (Built-in Test Equipment) computer during takeoff roll. They removed an access panel under the leading edge of the left wing; found some paint chips between the 'sensor' and the 'target' that they believed would have caused erroneous report. After clearing the debris; they ran the flaps through all settings several times without any indication of defect; closed the panel up; signed us off; and away we went.The maintenance personnel were knowledgeable about what to check and how to check it and wasted no time in getting to the problem and fixing it. Shortly after parking; I suspected a sensor problem was the cause (not actual flap malfunction); so I told the station that we would not plan on deplaning right then. The passengers were very cooperative and understanding. As the investigation and repairs progressed; I made at least four rather long PAs in which I relayed the known issues at the time and what next steps might be coming. Probably nothing could be done to avoid this. TAKEOFF CONFIG is not an alarm that I ever expected to get at high speed but I believe I made the right call to reject given the circumstances.

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.