Fix: This device is already in use (0X00000927) on Windows

What's actually causing this error?

I've seen 0X00000927 (NERR_CommDevInUse) more times than I care to count. It's Windows telling you some app has grabbed hold of a COM or LPT port and won't let go. The classic scenario: you're plugging in a serial barcode scanner, a legacy printer on LPT1, or even a USB-to-serial adapter, and bam — the port's "already in use."

Had a client last month whose entire label printing setup stopped working because an old inventory app had a background process that grabbed COM3 and never released it. Took me five minutes to find the culprit with Process Explorer.

Most of the time, the fix is straightforward. You just need to figure out which process owns the port and either kill it or configure Windows to not reserve it by default. Let's walk through the three most common causes and how to squash each one.

Cause 1: A background app or service is hogging the port

This is the #1 cause. Some program — maybe a POS system, a label printer utility, or even a Windows service — has the port open. The port's not physically used, but the OS treats it as locked.

The fix: Find and kill the process with Process Explorer or Resource Monitor

1. Download Process Explorer from Microsoft (it's free, no install needed).
2. Run it as Administrator (right-click, Run as administrator).
3. Hit Ctrl+F and type the port name, e.g., COM3 or LPT1. Process Explorer will show you which process has that handle.
4. If you see a process like cmd.exe, posapp.exe, or spoolsv.exe — right-click it and choose Kill Process.

If you can't run Process Explorer, use Resource Monitor (built into Windows):

• Press Win+R, type resmon, hit Enter.
• Go to the CPU tab, expand Associated Handles, search for COM or LPT.
• You'll see the process. Right-click it and End Process.

After killing it, try your device again. If the error's gone, you've found the culprit. Next step is to figure out why that app grabbed the port — maybe it's running as a startup service you can disable.

Cause 2: Windows port reservations (especially with POS systems)

Some older POS or serial device software installs a service that reserves COM or LPT ports at boot. Even when you close the app, the port stays locked. I've seen this with OPOS (OLE for Retail POS) drivers and some custom serial utilities.

The fix: Disable the reservation service or remove the driver

1. Press Win+R, type services.msc, hit Enter.
2. Look for anything with a name like Port Reservation, COM Reservation, or the name of your POS software.
3. Right-click it, choose Properties, set Startup type to Disabled, then click Stop.
4. Reboot your machine.

If that doesn't work, try removing the port driver entirely:

devmgmt.msc

Open Device Manager, expand Ports (COM & LPT), right-click the port showing the error, select Uninstall device. Check Delete the driver software for this device if prompted. Then reboot — Windows will reinstall a clean driver.

I've fixed a dozen POS systems this way. The reservation service usually re-creates itself if you don't uninstall the driver too, so do both steps.

Cause 3: A hung print job or print spooler lock

If you're dealing with an LPT port (parallel printer), a stuck print job can tie up the port. The spooler doesn't always clear it automatically. I've seen this on Windows 10 and 11, especially with older dot-matrix printers connected via USB-to-parallel adapters.

The fix: Clear the print queue and restart the spooler

1. Press Win+R, type services.msc, hit Enter.
2. Scroll to Print Spooler, right-click it, choose Stop.
3. Open File Explorer and go to C:\Windows\System32\spool\PRINTERS. Delete everything in that folder.
4. Back in Services, right-click Print Spooler again and choose Start.

Now try printing or using the device. If it works, you had a stuck job. To prevent this, you can also set your printer to Print directly to the printer (bypassing the spooler) in printer properties:

• Go to Settings > Bluetooth & devices > Printers & scanners.
• Click your printer, then Printer properties.
• Go to the Advanced tab, select Print directly to the printer.

This works great for LPT ports but might cause issues with network printers — your mileage may vary.

Quick-Reference Summary Table

One last thing: if you're still stuck after all these, check your USB-to-serial adapter drivers. I've had cases where a cheap Prolific chipset adapter gets assigned multiple COM ports, causing conflicts. Update the driver from the manufacturer's site, not Windows Update. That's usually the nail in the coffin for this error.