404 Override Not Working

Viewing the Core code, setting a custom 404_override in ./config/routes.php does not change how the show_404() method is used.

The override is used for when your Controller can not be located. It will then load the Controller that is set in the config value.
This is not what is happening in your case, because CI can locate the controller.

Look at the docs https://codeigniter.com/user_guide/gener...l#show_404

What you need todo is create your own show_404 method or redirect to your custom Notfound controller instead of calling show_404 and pass along any needed arguments

Thanks for that will look into it.

You can easily test this.

Open ./system/core/CodeIgniter.php
Go to line 447 if($e404)
Place a die after the opening {
Navigate to a thread that can not be found via your Viewthread Controller
If it dies, then the required Controller could not be located. Else it was located and you need something other than show_404