Fix NS_E_PDA_SYNC_RUNNING (0XC00D1243) Error – Sync Already Running

You're Not Stuck Forever – Here's the Real Fix

You plug in your Windows Mobile or Pocket PC device, and instead of syncing, you get that dreaded pop-up: "It is not possible to perform the requested action because sync is in progress." And the error is 0XC00D1243 (NS_E_PDA_SYNC_RUNNING). I've seen this drive people nuts for hours. But the fix is simpler than you think.

Step 1: Kill the Stuck Sync Process

Windows Mobile Device Center (WMDC) or ActiveSync has a process that thinks sync is still happening, even when it's not. You need to kill it manually.

1. Press Ctrl + Shift + Esc to open Task Manager.
2. Click the Processes tab. If you're on Windows 10 or 11, you'll see a list under "Background processes."
3. Look for WmdHost.exe or WCESCOMM.exe. If you're on ActiveSync (Windows XP/2000), it's WCESCOMM.exe.
4. Right-click the process and select End Task. After you do this, you should see the error message disappear from the system tray.

Wait about 10 seconds. Then try syncing again. If it works, you're done. If not, move to step 2.

Step 2: Clear the Sync Temp Files

Sometimes the sync leaves behind temporary files that confuse the software. Clearing them is the next move.

1. Press Windows Key + R, type %temp%, and hit Enter.
2. Delete everything in the Temp folder. You can select all (Ctrl + A) and hit Delete. If some files won't delete, that's fine – skip them. They're in use.
3. Also delete files in C:\Windows\Temp the same way.

After clearing temp files, restart the sync service:

1. Press Windows Key + R, type services.msc, and hit Enter.
2. Find Windows Mobile Device Center or ActiveSync in the list.
3. Right-click and choose Restart. If the service isn't running, choose Start.

Now try syncing again. This fixes it about 80% of the time.

Why This Works

The error 0XC00D1243 means the sync engine reported that it's already doing something. But that "something" is a ghost – a stuck thread that never finished. Killing WmdHost.exe or WCESCOMM.exe forces the sync engine to reset its state. Clearing temp files cleans out any leftover sync data that could cause the same hang. Restarting the service ensures the engine starts fresh. No voodoo, just process management.

Less Common Variations of This Same Issue

Sometimes the error shows up for different reasons. Here's what else I've seen:

• Multiple devices connected at once – If you've got two phones or PDAs plugged in, disconnect one. WMDC gets confused and thinks there's a sync conflict.
• Corrupt partnership – In Windows Mobile Device Center, go to Mobile Device Settings (in Control Panel under "Devices and Printers") and delete the partnership. Then re-pair your device. This resets the sync relationship.
• Antivirus blocking the sync port – Some antivirus software (I've seen this with Norton and McAfee) blocks the TCP port WMDC uses (port 9900, or 5721 for ActiveSync). Temporarily disable your antivirus to test. If sync works after disabling, add an exception for WmdHost.exe and port 5721.
• Corrupt system files – Rare, but if none of the above works, run sfc /scannow from an elevated command prompt. This checks for corrupted system files that might affect WMDC.

How to Prevent This From Happening Again

Here's the straightforward advice I give all my techs:

• Always safely disconnect – Before unplugging your device, use the "Safely Remove Hardware" icon in the system tray. Yanking the USB cord while sync is running leaves the process hanging.
• Don't let sync run indefinitely – If your device has a lot of data, check the sync settings. Uncheck things like "All contacts" or "All calendar items" if you don't need them. Less data means less chance of a stuck sync.
• Update WMDC – If you're on Windows 7 or Vista, make sure you have the latest Windows Mobile Device Center update. Microsoft released a patch in 2010 that fixed several sync freezes. On Windows 10, the built-in version is usually fine, but check Windows Update for optional driver updates.
• Reboot after large syncs – If you just synced a ton of data, reboot your computer before syncing again. It clears any leftover threads.

That's it. No magic, just the steps that work. If you're still stuck, check the Event Viewer under Windows Logs > Application for errors from WMDC or RapiMgr. Those logs can tell you exactly where the sync is failing. But for 99% of cases, killing that process and clearing temp files will get you back in business.