"This error indicates that the hand control is unable to communicate with the motor control processors located inside the mount. One source for this error is faulty power, either low voltage (weak batteries) or a loose connector."
http://www.nexstarsite.com/OddsNEnds/NoResponse.htm
"Also, there is a problem on some AS-GT mounts regarding insulation between the bottom of the motor control board and the mount. Peter Bruce has written an article describing the problem and the fix." - This may also be the cause. Look at the link above to get more info.