Client requests receive a "503: Service Unavailable" error
Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1
The 503: Service Unavailable error is a noncustomizable error that means the server is currently unable to process the request. There are many reasons why the server might not be able to process a request. For example, the administrator might have shut down an application pool to perform maintenance. Another possible cause of the 503 error is that the application pool queue was already full when the request arrived.
If the error was generated because the application pool queue was full at the time of the request, you will see error code 503 along with the message "QueueFull" when viewing the Httperr.log file. The Httperr.log file is located in the %Systemroot%\System32\LogFiles directory.
To remedy this problem, increase the application pool queue length limit as detailed in Configuring Application Pool Queue-Length Limits.