Using ScanOST to Repair OST Files

Like the ScanPST utility to repair .pst files, Microsoft provide an OST Integrity Check Tool, scanost.exe, to analyze and repair Offline Folder files, i.e. .ost files.

The utility will normally be in "C:\i386" and "C:\Program Files\Common Files\System\MSMAPI\1033". You are likely to have the same version in both folders. I've included information below on a version I found on a system running Outlook 2003 on a Windows XP Professional Service Pack 2 system. The copies in "C:\i386" and ""C:\Program Files\Common Files\System\MSMAPI\1033" were identical.

C:\Program Files\Common Files\System\MSMAPI\1033
Size:50.6 KB (51,912 bytes)
MD5 Sum4f692599e51a13bccb5e78afad9193d5

Close Outlook before running the scanost.exe utility. When you run the utility, if Outlook is still running at the time, you will see the warning "Please quit Outlook before performing a scan with the OST Integrity Check Tool." Even if you don't see Outlook in the taskbar at the bottom of your screen, it may be running in the background. This can occur if Outlook doesn't close properly. You can locate and close such instances of Outlook by running the Task Manager. To find and close instances of Outlook with the Task Manager, take the following steps:

  1. Hit the "Ctrl", "Alt", and "Del" keys simultaneously.
  2. Select "Task Manager".
  3. Click on the "Processes" tab.
  4. Click on "Image Name", which is a column header. That will put the process names in alphabetical order.
  5. Locate all instances of Outlook and close them by clicking on them and then selecting "End Task" for each one. When you get the warning "Terminating a process can cause undesired results including the loss of data and system instability", click on "Yes" to kill the process anyway. Typically you will see a CPU value of zero for such stuck processes and there is nothing you can do but kill them.
  6. Then click on "OK" for the OST Integirty Check tool to proceed.

A scan will be conducted for the .ost file belonging to the account under which you have logged onto the system. You will see options to "Scan all folders" or "Choose folder to scan (select this option if time or connection speed is a consideration)". Normally you should take the default value of "Scan all folders". Leave "Repair errors" checked to have it repair any errors it finds and then click on "Begin Scan". When the tool has completed analysis of the file, you should see the message "The scan is complete. See the OST Integrity Check message in your Deleted Items folder for more details. If you look in the Deleted Items folder in Outlook, you should see a message containing information about the results of the scan.


  1. File Extension Details for .OST
    FILExt - The File Extension Source
  2. Repair an .ost or .pst file in Outlook
    Microsoft Office Assistance