Nagios is a great product for monitoring IT infrastructure. Since it is an open source product users can develop there own plugins to monitor what ever parameter they like to monitor.
If you are using a plugin to monitor something and it shows “Return code of 13 is out of bounds” on Nagios Screen, the problem in most cases is the permission not set properly.
Change Permission on the required plugin. In my case, plugin is check_port.
# service nagios restart
# systemctl restart httpd.service