When I go to adjust the mirror using the knob, the mirror goes up down but not left right. Is this the sure sign of mirror failing?  I know this worked before. Codrivers mirror work well.

When you say the mirror failing, its a fairly involved system. There are 4 wires running to a hub behind the mirror glass - these control 2 motors in each direction. 

These are controlled by the door control module.

 

To correctly diagnose youll need to remove the glass and the hub and check for the correct voltage at each of the 4 pins when the mirror is moved. The opposing pin should switch to ground. If all of these test okay youll need to then check the motors themselves to see if they function. This should put you in the right direction. 

 

A fault scan of the door module will also give a better understanding and id expect a open/short code to be stored. 

I went to the dealer. They said something about some potentiometer failed inside. They offered to change this part for 400 eur.  I guess I can live without this.

You can still move the mirrors manually anyway.