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PCI Express device serial number conflicts cause device enumeration errors in Windows

This article provides a workaround for device enumeration problems caused by PCI Express devices that report duplicate serial numbers due to a hardware defect.

Applies to

  • Windows 10
  • Windows 11
  • Windows Server 2016 and later versions

Symptoms

Consider the following scenario:

  • A computer has multiple PCI Express devices installed.
  • Two or more of these devices report the same PCI Express device serial number through the Device Serial Number Extended Capability.

In this scenario, some devices don't start correctly, some devices are assigned the wrong driver, and some devices exhibit unexpected behavior. You might see one or more of the following problems:

  • Devices appear interchanged or misidentified in Device Manager.
  • A device fails to start with an error code in Device Manager.
  • Driver updates are applied to the wrong device.
  • Device-specific settings or configurations are applied inconsistently across reboots.

Cause

PCI Express devices can optionally implement the Device Serial Number Extended Capability (as defined in the PCI Express specification). This capability provides a unique 64-bit serial number that Windows uses to consistently identify and track individual devices, even when you move them to different slots or when the system reboots.

Windows relies on this serial number to maintain a stable association between a physical device and its software configuration (driver settings, device properties, and so on).

Some hardware implementations contain a defect where multiple devices of the same model report an identical serial number. This defect violates the PCI Express specification, which requires that each device serial number be unique. When Windows encounters duplicate serial numbers, it can't reliably distinguish between the affected devices, which leads to the symptoms described earlier.

Workaround

Important

This article contains information about how to modify the registry. Make sure that you back up the registry before you modify it. Make sure that you know how to restore the registry if a problem occurs. For more information about how to back up, restore, and modify the registry, see How to back up and restore the registry in Windows.

Warning

Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall the operating system. Microsoft can't guarantee that these problems can be solved. Modify the registry at your own risk.

To work around this issue, disable PCI Express Device Serial Number support system-wide by setting the PCI_SYS_HACK_DISABLE_EXPRESS_SERIAL_NUMBER_SUPPORT hackflag. This change causes the Windows PCI driver (pci.sys) to ignore the Express Serial Number capability for all devices.

When you set this hackflag, Windows no longer uses the PCI Express Device Serial Number to identify devices. Device identification falls back to other methods, such as bus location, which might reset device settings if you move devices to different slots.

To apply this workaround:

  1. Select Start, type regedit in the search box, and then select regedit in the results list.

    If you're prompted for an administrator password or for confirmation, type the password, or select Continue.

  2. Locate the following registry subkey, and then select it:

    HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\PnP\Pci

  3. If the HackFlags registry entry doesn't exist, follow these steps:

    1. On the Edit menu, point to New, and then select DWORD (32-bit) Value.
    2. Type HackFlags, and then press ENTER.
    3. Right-click HackFlags, and then select Modify.
    4. In the Value data box, type 0x10000, select Hexadecimal in the Base area, and then select OK.
    5. Exit Registry Editor.

  4. If the HackFlags registry entry already exists, the value might contain other hackflags that are already in use. In this case, combine the new flag with the existing value using a bitwise OR operation, rather than replacing it. Follow these steps:

    1. Right-click HackFlags, and then select Modify.
    2. Select Hexadecimal in the Base area.
    3. Note the current Value data (for example, 0x600).
    4. Calculate the new value by performing a bitwise OR of the existing value with 0x10000. For example, if the current value is 0x600, the new value is 0x10600.
    5. Type the new value in the Value data box, and then select OK.
    6. Exit Registry Editor.

    Tip

    You can use the Windows Calculator in Programmer mode to perform the bitwise OR. Select HEX format, enter the existing hex value, select OR, enter 10000, and press = to get the combined result.

  5. Restart the computer for the change to take effect.

How to verify the workaround is applied

After restarting the computer, verify the hackflag is active by checking the registry value:

  1. Open Registry Editor and go to:

    HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\PnP\Pci

  2. Double-click HackFlags and confirm that the value, when viewed in hexadecimal, includes the 0x10000 bit. For example, a value of 0x10000, 0x10200, 0x10600, or any value where bit 16 is set confirms the flag is active.

More information

  • The hackflag value 0x00010000 corresponds to PCI_SYS_HACK_DISABLE_EXPRESS_SERIAL_NUMBER_SUPPORT in the Windows PCI driver.
  • This setting affects all PCI Express devices. You can't apply it to individual devices.
  • Use this workaround only for systems that have hardware with a known duplicate serial number defect. Don't use it as a general-purpose configuration change.

Data collection

If you need assistance from Microsoft support, collect the information by following the steps mentioned in Gather information by using TSS for deployment-related issues.

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