liftmaster 8500 Gors down to ground stops and goes up

Liftmaster 8500 working until now.  when door closes it goes to the ground, the normal closed position, but then goes back up.
The yellow led blinks 12 times meaning "door compensation" what ever that really means. 

Tried to reset position, but that does not work right.  black button door up works, but then pressing the remote to send door down which sets the up position, sends door down for a second then stops where it should keep going.

The light sensor is good, cable tension switch is good.   From what I have read, it is the absolute encoder. 

Like to know thats it or might be something else. not that old.

A faulty absolute encoder can be the reason, at least the chances are high. Another point that is less likely, but still possible, is an issue with the programmed door travel limits. Incorrect limits could confuse the opener and cause it to retract after reaching the bottom.