Fix 0XC0262400: Child Device Already Connected

When This Error Shows Up

You plug in a second monitor via HDMI, DisplayPort, or USB-C dock. Windows throws the error 0XC0262400 with the message about a child device already connected. You might see this when using a laptop with a docking station that has multiple video outputs, or when hot-plugging a monitor into a GPU that has another display already attached through an MST hub.

What's Going On

The error means Windows thinks the physical port on your graphics adapter is already in use. The culprit here is almost always a ghost device—a leftover entry in the graphics adapter's child device list. This happens when you disconnect a monitor without properly ejecting it (like yanking a USB-C cable), or when a dock or MST hub doesn't fully release its connection. The system still sees that port as occupied, so any new device gets blocked.

Fix: Step by Step

1. Power Cycle the Display Chain

Don't bother with software fixes first—power cycling clears the stale connection faster than anything else.

Shut down your PC completely—not sleep, not hibernate.
Unplug all monitors, docks, and USB-C cables from the computer.
Disconnect power from the monitors and dock for 30 seconds.
Reconnect everything: power first, then cables, then boot the PC.
Plug in the problem monitor last.

If you're using a Thunderbolt dock, also unplug the dock's power brick for a full minute. These docks cache device states and a hard reset is the only way to clear them.

2. Remove Ghost Devices from Device Manager

If power cycling didn't work, we force Windows to forget the phantom connection.

Open Device Manager: Win+X then M.
From the View menu, select Show hidden devices.
Expand Display adapters. Right-click your GPU and choose Uninstall device. Check the box that says Delete the driver software for this device if it appears.
Now expand Monitors. You'll likely see a grayed-out monitor listed there—that's your ghost. Right-click and Uninstall device.
Also check Universal Serial Bus devices and Other devices—sometimes a USB-C dock shows up there as a phantom.
Reboot. Windows will reinstall the GPU driver and rediscover monitors.

3. Clear the Graphics Adapter Child List via Registry

This is the nuclear option—use it when the first two steps fail. It targets the registry key where Windows stores the list of connected child devices for each GPU.

Back up your registry first. Seriously, don't skip this.

Open Registry Editor: Win+R, type regedit, hit Enter.

Navigate to:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\GraphicsDrivers

Look for a subkey named ChildDeviceList. It may be empty or have one or more entries.
Right-click ChildDeviceList and export it as a backup (.reg file) to your desktop.
Delete the ChildDeviceList subkey entirely.
Close Registry Editor and reboot.

4. Update or Roll Back GPU Driver

Sometimes the driver itself gets confused. If you recently updated your GPU driver, roll back:

Device Manager > Display adapters > Right-click GPU > Properties.
Driver tab > Roll Back Driver if available.
If roll back isn't there, download the previous driver from NVIDIA, AMD, or Intel's site. Use the DDU tool (Display Driver Uninstaller) in Safe Mode to nuke the current driver, then install the older version.

If It Still Fails

Check your cable and monitor. Some cheap USB-C to HDMI adapters don't properly report their connection status—they leave a phantom link when unplugged. Replace the adapter with a certified one (look for USB-IF or VESA certification). Also try a different port on your GPU or dock. If you're on a laptop with a dedicated GPU and an integrated one, make sure the external monitor is plugged into the port wired to the correct GPU—manufacturers sometimes label them.

Still stuck? Boot into Safe Mode and see if the error repeats. If it doesn't, some third party software (like monitor color calibration tools or remote desktop services) is holding the connection. Use msconfig to do a clean boot and isolate the culprit.