can bus bus-off manual or automatic

I am working on a project where i need to analyze the error dynamics of the CAN bus network. I have implemented bus off attack in the CAN bus prototype using Sparkfun CAN bus shield and Arduino uno.

The TEC of the CAN is becoming zero after bus-off. Can some one tell me if the transformation bus-off to error active state is done automatically or do we need to do manually ?

if automatic can anyone please tell how to change the transformation to manual( Arduino code/function)?

if manual can anyone please tell me why i am getting zero TEC after reaching bus-off?

Thanks in advance