Service Unavailable HTTP Error 503. The service is unavailable.

Generally, this error occurs if the application pool that is associated with the web application doesn't start. To troubleshoot this issue, follow these steps:
In Event Viewer, view the System log to find errors from the Microsoft-Windows-WAS source. An event is frequently logged in the System log if the application pool fails to start.
If you don't find any relevant events in the System log, search for relevant entries in the HTTPERR log file. The httperr1.log file is located in the following system folder:
c:\windows\system32\logfiles
In the file, search on "503" to locate any relevant information about why the application pool failed. For example, you may see an entry that resembles the following:
2011-12-08 18:26:42 ::1%0 6721 ::1%0 443 HTTP/1.1 GET /owa 503 3 N/A SharePoint+Web+Services+Root
The log entry should list the site you are trying to browse and the application pool that is being used. Make sure that the correct application pool is being used for the site.
For example, you try to browse the "/owa" website by using the Sharepoint Web Services Root application pool. The OWA application is supposed to use the MSExchangeOWAAppPool application pool. However, there is a misconfiguration in Internet Information Services (IIS). To resolve this issue, you would check the Application Pool for the site and for the site bindings to make sure that IIS is configured correctly.
Open IIS Manager MMC. To start IIS Manager from the Administrative Services console, follow these steps:
On the Start menu, click All Programs, click Accessories, and then click Run.
In the Run text box, type control panel, and then click OK.
In the Control Panel window, click Classic View, and then double-click Administrative Tools.
In the Administrative Tools window, double-click Internet Information Services.
Check the bindings for each of the websites to make sure that they match the default settings in the following table.
Website Type Host Name Port IP Address Binding Information
Companyweb http Companyweb 80 * Port 80
Default Website HTTP

http

net.tcp

net.pipe

net.msmq

msmq.formatname

https

https

80

80









443

443 *

127.0.0.1









*

127.0.0.1



808:*

*

Localhost

localhost

SBS Client Deployment Applications http

http

SBSConectComputer

80

* *:80:connect
SBS SharePoint https 987 *
SharePoint Central Administration v4 http 18000
Check each Application Pool value to make sure that the settings are correct. The following table contains the default settings for the standard SBS application pools.
Application Pool .NET Framework Version Identity Associated Applications
DefaultAppPool V2.0 ApplicationPoolIdentity /rpc

/rpcwithcert
MSExchangeAutodiscoverAppPool V2.0 LocalSystem /Autodiscover

/Autodiscover/bin

/Autodiscover/help
MSExchangeECPAppPool V2.0 LocalSystem /ecp
MSExchangeOWAAppPool V2.0 LocalSystem /owa
MSExchangeOWACalendarAppPool V2.0 LocalSystem /owa/Calendar
MSExchangePowerShellAppPool V2.0 LocalSystem /PowerShell
MSExchangeSyncAppPool V2.0 LocalSystem /Microsoft-Server-ActiveSync
MSExchangeServicesAppPool V2.0 LocalSystem /EWS

/EWS/bin
SBS Sharepoint AppPool V2.0 <Domainname>\spwebapp Root Application for SBS SharePoint

Root Application for Companyweb
SBS Web Applications application pool V4.o NetworkService /webhelp

Root Application for Default Website
SBS Web Workplace AppPool V4.0 NetworkService /Remote

The following documented issues commonly cause an HTTP 503 "service unavailable" error.

Issue 1: SharePoint AppPool identity is invalid