I am curious how to calculate the error codes and how exactly it has the car?
is there a scheme somewhere or?for example how is it known that 1121 is about the throttle potentiometer?

nobody knows? where do these codes come from?

Hi
The error codes is part of a vechile manufacturers standard libary for ECU error identification, read via OBD ports.
Many codes are shared between manufacturers for truying to standarize although not all.
On the forum there are quite a few Fiat code libraries and you can also google for the code with manufacturer in mind and also see possible solutions.

I thought all the codes were written somewhere