Enable Trace Logging for Failed Requests (IIS 7)

Open IIS Manager and navigate to the level you want to manage. For information about opening IIS Manager, see Open IIS Manager (IIS 7). For information about navigating to locations in the UI, see Navigation in IIS Manager (IIS 7).

In the Connections pane, click Sites.

In Features View, select the site for which you want to enable trace logging.

In the Actions pane, under Configure, click Failed Request Tracing.

In the Edit Web Site Failed Request Tracing Settings dialog box, select Enable to enable logging for this site.

In the Directory text box, type the path where you want to store the log files or click the browse button (…) to find a location on the computer. The default is %SystemDrive%\inetpub\logs\FailedReqLogFiles.
As a best practice, log files, such as log files for failed request tracing, should be stored in a directory other than systemroot.

In the Maximum number of trace files text box, type the maximum number of trace log files that you want to keep, and then click OK.