Contributed By: Cliff Hobbs [MVP SMS]
As part of notifying users of operations being performed, the Automatic Updates client writes several different Event IDs to the System Event Log. The following list (reproduced from the Software Update Services Deployment White Paper), is a list of Event IDs and their meaning:

 

Event ID	Description
16	Unable to connect: The Automatic Updates client is unable to connect to the update service (Windows Update or the server running SUS) and therefore cannot download and install updates according to the set schedule. Windows will continue to try to establish a connection.
17	Install ready – no recurring schedule: Downloaded updates that are ready to install are listed in the event. To install the updates, an administrator needs to log on to the computer, where they see the notification area icon and install the updates.
18	Install ready – recurring schedule: Downloaded updates that are ready to install are listed in the event. Additionally, the date and time for the scheduled installation of those updates is listed.
19	Install success: Successfully installed updates are listed.
20	Install failure: Updates that failed to install are listed.
21	Restart required – no recurring schedule: To complete the installation of the listed updates, the computer must be restarted. Until the computer has been restarted, Windows cannot search for or download new updates.
22	Restart required –recurring schedule: To complete the installation of the listed updates, the computer will be restarted within five minutes. Until the computer has been restarted, Windows cannot search for or download new updates.