What is an HTTP 503 error?

Answer: 503 is one of many HTTP status codes that can be sent from a web server to a web browser. It is an error code that means "service unavailable" and is used by both Apache and Microsoft IIS web servers.

A webpage may display a 503 error for several different reasons. For example, if a web server is overloaded with traffic, it may not be able to handle new visitors. When you access a webpage on an overloaded web server, you may be greeted by a 503 error message instead of seeing the original webpage content. Similarly, if a server is processing a large amount of data, such as memory or CPU-intensive script, there may not be enough resources to process incoming requests. This may also produce a 503 error.

The error message "503 Service Unavailable" may also indicate a service is simply not running on a web server. For example, if you try to access Microsoft Outlook Web Access (OWA) on a Microsoft Exchange Server, but the web access service is not running, you may see a 503 status code. This may happen if either the System Attendant or Information Store services are not running on an Exchange server.

503 status codes are temporary, meaning they will stop appearing once a server can handle incoming requests and the necessary services are running. Therefore, if you come across a 503 error on a website, chances are, if you wait a few minutes and try loading the webpage again, it will work correctly. If several hours go by and you still see a 503 error, it may be helpful to email the webmaster (if possible) to notify him or her of the error.

Published: May 22, 2014 — by Per Christensson