codeigniter 3.0 custom 404 not found error page

You need to set in application/config/routes.php the following route

$route['404_override'] = 'my404';

Then you need to create a new controller in your controllers directory (application/controllers/)

<?php 
class My404 extends CI_Controller 
{
 public function __construct() 
 {
    parent::__construct(); 
 } 

 public function index() 
 { 
    $this->output->set_status_header('404'); 
    $this->load->view('err404');//loading in custom error view
 } 
} 

And create an err404 view. Thats all!

Refer :Reserved Routes

It's not working in codeigniter 3, I overwrite the default view (application/views/errors/html/error_404.php) and added my custom view, now show_404() is working fine.