Air crash investigators say the Sriwijaya plane crash last month was due to an imbalance in engine thrust that led the plane into a sharp roll and then a final dive into the sea.
The findings are part of a preliminary report into the disaster that killed 62 passengers and crew.
When the 26-year-old Boeing Co 737-500 plane reached 8,150 feet after take-off, the left engine throttle lever moved back while the right lever stayed in its original position.
"At the moment we know the left autothrottle moved back, but we don't know whether it is broken. Because two of the levers (autothrottles) showed 'abnormality'. The left moved far back, and the right did not as though it was stuck. So, we don't know whether it is the left or the right that is broken. This is what we cannot confirm until today - is the autothrottle broken?"
There had been two prior problems reported with the autothrottle system that automatically controls engine power based on maintenance logs.
But the issue was rectified on January 5, four days before the crash, Indonesia's National Transportation Safety Committee said.
A working autothrottle is not required for a plane to be dispatched as pilots can control the thrust levers manually with their hands.
The Sriwijaya accident was Indonesia's third major airline crash in just over six years.
Divers are still searching for the plane's cockpit voice recorder which could help investigators understand the actions taken by the experienced pilots.
The preliminary report did not list the contributing factors to the crash.
That will require further investigation.