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Find and Delete Extend.dat

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› Tutorials › Find and Delete Extend.dat

Last reviewed on May 7, 2014     9 Comments

When you receive and error that Outlook can't find and add-in, the usual cause is that you uninstalled it but Outlook didn't delete the file it uses to keep track of installed add-ins. This file is extend.dat and is found in the (hidden) local app data file under your user account. It's in the same folder as your Outlook data files.

You can quickly jump to it by typing or pasting the following command in the Start Search field in Windows 7 or Vista.

%localappdata%\microsoft\outlook

Delete it and restart Outlook. Outlook will rebuild the extend.dat field and the errors should go away.

In most cases, you'll need to restart Outlook for any changes to take effect.

Tip: when you use these commands to open hidden files and folders, you can see the contents of the folder without changing your settings for Hidden files and folders.

Windows XP Location

If you are using Windows XP, extend.dat (and the data files) are at

%USERPROFILE%\Local Settings\Application Data\Microsoft\Outlook

Type or paste it into the Start menu, run command (you can also use Windows key + R to open the run command) and press Enter.

Open the Add-ins Dialog

To view the installed add-ins, go to Outlook 2007's Tools menu and select Trust Center, then Add-ins. In Outlook 2010, you'll go to File, Options, Add-ins. The list of installed (and the active or disabled state) is on this screen. Click Go at the bottom of the screen to open the dialog which will allow you to enabled or disable the add-ins.

In older versions of Outlook, go to Tools, Options, Other, Advanced options, Add-in Manager and COM Add-ins.

Delete Extend.dat Video Tutorial

More Information

For more information on shortcuts to the file locations, see Where Are My Files? (Outlook-tips.net)

For more information on the files Outlook uses and which need backed up, see Outlook 2007 Backup and File Locations

Find and Delete Extend.dat was last modified: May 7th, 2014 by Diane Poremsky

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About Diane Poremsky

A Microsoft Outlook Most Valuable Professional (MVP) since 1999, Diane is the author of several books, including Outlook 2013 Absolute Beginners Book. She also created video training CDs and online training classes for Microsoft Outlook. You can find her helping people online in Outlook Forums as well as in the Microsoft Answers and TechNet forums.

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curtis (@guest_203097)
November 24, 2016 12:22 pm
#203097

How to query version of pst file because outlook2002 questions opening it? Warning posts contact admin because pst incompatible or wrong. Some pst that successfully open don't jive with its size, missing mssgs, or empty with deleted icon empty too.

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Diane Poremsky(@diane-poremsky)
Author
Reply to  curtis
November 24, 2016 11:06 pm
#203103

2002 won't report the pst type (ansi or unicode) - it can only tell you that it's not compatible. Outlook 2003 and above will identify the pst type. If the pst is over 2GB, 2002 won't be able to open it.

Are you sure the pst is the wrong type instead of corrupt?

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curtis chun (@guest_203086)
November 23, 2016 5:38 pm
#203086

i use office xp but outlook will not open archive pst files from my employment outlook i copied and brought home to my pc.

i do not understand whether archived pst files are not ok or my home xp os pro sp3 is not compaible with xp office loaded

some pst files do open but appear incongruent with the pst filesize 1.9gb where limit is 2gb in 2002 office version.

for each pst archived there is an accompanying file with the pst file called extend.dat, dunno whether i must delete it 1st before opening it in outlook (not import pst for it does not work! causes crash corrupt)

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Diane Poremsky(@diane-poremsky)
Author
Reply to  curtis chun
November 24, 2016 11:34 pm
#203104

extend.dat isn't important - it is a support file that shows what addins are installed.

While we say the the pst size limit is 2GB, it's actually 1.9 GB, so it sounds like the pst are at the max if they are ansi pst files. They could be slightly oversized and corrupt as a result. Try installing one of the trials for pst repair - they will let you look in the pst. Any of the utilities (with free trials) on this page should work - https://www.slipstick.com/problems/pst-repair/repair-a-2gb-personal-folders-or-offline-folders-file/

What version of Outlook did you use at work? either the pst are corrupt or created in the newer versions.

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Una (@guest_185652)
September 5, 2014 7:07 pm
#185652

I use Outlook 2003 on my PC. I deleted the file & restarted outlook, but am getting the alert, Intel Wireless Bluetooth: & the programme has, 'stopped working'. Do I follow the same procedure?

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Reply
Diane Poremsky (@guest_185707)
Reply to  Una
September 9, 2014 1:01 am
#185707

Go to File, Options, Other, Advanced, Addins see if there is a Bluetooth addin listed - if so, uncheck it. If the addin didn't exist, deleting extend.dat would remove it from outlook, but since it sounds like you deleted extend.dat already, the addin may be installed.

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Carl (@guest_182013)
February 16, 2014 11:47 pm
#182013

didn't find any info on 2013

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Diane Poremsky (@guest_182014)
Reply to  Carl
February 17, 2014 12:44 am
#182014

Extend.dat is in the local app data folders for all outlook versions, the operating system controls the actual path to that folder. Because Outlook 2013 is only supported on Windows 7/8, extend.dat is in the Win7 location - %localappdata%\microsoft\outlook

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-2
Reply
Paul G (@guest_178213)
July 16, 2013 7:31 am
#178213

Thank you!

2
-2
Reply

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