B737-700
Flight Crew Last 90 Days 310
Human-Machine Interface; Time Pressure
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Returned To Gate; Flight Crew Rejected Takeoff; General Maintenance Action
Aircraft; Human Factors; Procedure
Takeoff was aborted at approximately 15 knots due to configuration warning horn.  A tripped circuit breaker was found and; since it was a flap configuration problem; we elected to return to gate and call Maintenance for a more detailed investigation.  A loose cannon plug was found on one of the flap indication circuits.  The cannon plug was cleaned and secured.  The circuit breaker was reset and flaps cycled to confirm safe function.  Logbook entries were made and flight departed without further problems.  I failed to verify flap position during before taxi checks.  Complacency and being in a hurry were the primary factors.  At first; I had concerns that I had also forgotten to do the thrust lever check prior to takeoff.  I could not see how I could have done the thrust lever check and not gotten the Takeoff Warning Horn.  But; the First Officer confirmed that I had indeed done the check.  My only reasoning as to why we did not get the warning horn was that I either did the thrust lever check too fast or I didn't go far enough with the levers to get the horn.  Whatever the cause; we came to the 100% conclusion that this check was performed.   Our saving grace was the last line of defense which was the throttle up on takeoff.  At approximately 15 knots; the Takeoff Warning horn sounded and we aborted the takeoff.  [I plan to] stop being complacent and to make a conscious effort to verify all procedures; which I will now be doing with great precision.  The other lesson learned is to make sure the thrust lever check is done slower and with more focus on the fact that it was definitely completed.  At the end of the day; the procedures we have in place saved us.   What upsets me is that I allowed the beginning safety nets to be lowered.  This should have been caught before we ever released the brakes for taxi.  Lesson learned!
B737 Captain reports receiving a Takeoff Warning Horn for flaps and rejecting the takeoff at 15 knots. A loose cannon plug in one of the flap indication circuits was found to be the cause.