Autopilot Drivestop error

[Roc]

Hi All,
Have an issue with my Autopilot (Raymarine ST4000 plus).  This weekend, while it was in Auto mode, the unit beeped and the words 'Drivestop' appeared on the screen.  Put it in standby and the message disappeared.  Now, when in Auto mode, and you push the buttons to turn the boat to port, it seems to work and the wheel moves.  However, when set to move the boat to starboard, nothing happens, the drive motor does not turn the wheel to the right.  Has anyone experienced this and if so, what did you find out? 

Thanks!!