The "503 Service Unavailable" error message is a generic error message that occurs when the server is unable to handle a request.

The good news is that these errors are quite common and can usually be easily fixed.

In our experience the "503 Service Unavailable" error message on a cPanel server is usually caused by of one the following:

File Usage Limit Exceeded

All Shared, Reseller and Multi Site hosting packages have a limit on the number of files you can have on a single cPanel account. When this limit has been exceeded for 7 days no further data is able to be written to the disk, resulting in the "503 Service Unavailable" error message.

You can fix the issue by removing some files to bring the usage back to within the limits. As soon as you have done this your website will start to operate normally.

Please check out this guide for more information on how to troubleshoot a high file usage on a cPanel account.

Incorrect PHP Hander

When migrating accounts from other web hosts we often see issues with an incorrect handler being set in the users .htaccess file. We use the CloudLinux PHP selector on all our serves to allow users to switch between multiple versions of PHP but if we are migrating a website from a host that uses cPanel/WHM's multi PHP manager it will create an incorrect entry in the users .htaccess file.

Take the following example:

# php -- BEGIN cPanel-generated handler, do not edit
# Set the ea-php56PHP

<IfModule mime_module>
AddHandler application/x-httpd-ea-php56___lsphp .php .php5 .phtml
</IfModule> 

# php -- END cPanel-generated handler, do not edit

The above code has set "ea-php56" as the PHP handler for the account, however as the server doesn't have "ea-php56" installed it results in a "503 service unavailable" error and the website isn't able to load.

To fix the issue you can comment out (or remove) the offending code from the .htaccess file and then set the PHP version correctly.

As soon as the offending code is commented out or removed the website will start to load again.