I'll give you one example.
You tell us you load a model named Code_model.
This model contains the find_code() function.
But now, you wonder why the line
in your controller doesn't work.
That tells me that you don't understand that you have to refer to the model, like this:
PHP Code:
$code = $this->code_model->find_code();
The CI documentation is very clear about this concept.