Why 503 Service Unavailable Error occurs in WordPress?

503 service unavailable error occurs in your site when your web server is unable to get any information from PHP script and this PHP script can be from a WordPress plugin, theme or wrong custom code snippet.

If the 503 error is caused by heavy usages, server failure, or DDoS attack, it is automatically fixed within a few minutes.

If a service unavailable on your website is caused by a bad code, then this 503 Service unavailable error remains until you delete that code from your site.