Using helpers on error pages

404 page is handled by system/core/Expeptions.php

to customize it extend it in application/core/MY_Exceptions.php

PHP Code:

class MY_Exceptions extends CI_Exceptions{

public function show_404($page = '', $log_error = TRUE){

$CI =& get_instance();

        $CI->load->helper('url');
//your custom code

    }
}