Problems with the system led to the pilots disengaging the autopilot in stormy weather in a bid to fix the situation, and then losing control of the Airbus A320-200, Indonesia's official National Transportation Safety Committee said.
Indonesian investigators say a fault in the rudder control system was a major factor in the crash in December last year of an AirAsia plane, with the loss of 162 lives.
Problems with the system led to the pilots disengaging the autopilot in stormy weather in a bid to fix the situation, and then losing control of the Airbus A320-200, Indonesia's official National Transportation Safety Committee said.
The year-long investigation into the crash found that actions by the crew in response to the malfunction also contributed to the disaster.
Investigators said the soldering on the system that helps to control the rudder's movement was cracked, leading it to send repeated warnings to the pilots.
In an attempt to reset the system, the autopilot was turned off, and the plane then started to roll, the report said.
"Subsequent flight crew action resulted in inability to control the aircraft," said the report. The plane went into a "prolonged stall condition that was beyond the capability of the crew to recover".
It added the flight data recorders did not indicate the weather had affected the aircraft.
Flight QZ8501 was en route from Surabaya to Singapore when it crashed into the Java Sea on December 28, 2014.