According to Indonesia's National Transportation Safety Committee’s final report, it was a a tiny soldered electrical connection in the top of the vertical stabilizer, or tail, that caused the triggering of an alarm, which in turn indicated a failure of the rudder movement control system.
The captain then actuated the on/off button to reset the system (Rudder Travel Limiter Units). The alarm stopped, but was set off again shortly after. Three times the pilot performed the same operation, but when the alarm sounded a fourth time, the captain tried something else. He attempted to reset the Flight Augmentation Computer that controls the aircraft, by removing and re-inserting its circuit breaker, however, in so doing, the captain inadvertently disengaged the autopilot, and the automatic stall protection.
At 38,000 feet, the aircraft stalled. For four minutes, inappropriate and contradictory actions (miscommunication) of both pilots, led to the non-recovery of the plane, and the AirAsia A320 crashing at sea.
The report points to shortcomings in the aircraft’s maintenance and lack of crew stall recovery training. The accident claimed the lives of the 162 people on board the A320.