Hi John

According to the book I have for the the your controller (Zapi sem-1 combi) the error code AL 6 is:

SERIAL ERROR#1 Wrong message or no message from serial tiller.

The tiller on that truck sends most of the information though 1 serial wire to the combi controller (pin 4A on the tiller to pin 5C on the combi controller, either there is a problem with the wire between the tiller and the controller or a supply to the tiller, a problem with the tiller head or a problem with the controller.
I don't have an error code AL 7 in this book but I have got a list of M.D.I. (display) codes somewhere I will see if I can find it.
Regards
Shane