Fix 0xC0262438 GPU driver mismatch after POST swap

Quick answer

Unplug the secondary GPU, boot into safe mode, run DDU to nuke all GPU drivers, shut down, install only the primary GPU, boot normally, install its driver, then add the second GPU.

Why this error actually happens

You're looking at 0xC0262438, which maps to ERROR_GRAPHICS_NOT_POST_DEVICE_DRIVER. What's actually happening here is a boot-order conflict. During POST (Power-On Self Test), the motherboard picks one GPU to initialize the display — usually the one in the primary PCIe slot. The Windows Display Driver Model (WDDM) then expects the driver you load to match that POST adapter's hardware ID. If you swapped GPUs, added a second card, or changed BIOS settings that switched which GPU posts, the driver you're trying to start doesn't match the POST adapter's PCI vendor/device ID. Windows refuses to load it and throws this error. I've seen this most often after upgrading from an older NVIDIA card to a newer one while leaving the old driver in place, or after adding a secondary AMD card to a system that originally posted with an Intel iGPU.

Fix steps

Remove the conflicting GPU. Shut down, unplug power, remove the secondary or newly added GPU. Leave only the card you want as primary (the one that should handle boot display).
Boot into Safe Mode. Press F8 during boot or use Shift + Restart from Windows login. Go to Troubleshoot → Advanced Options → Startup Settings → Enable Safe Mode.
Run DDU (Display Driver Uninstaller). Download the latest version on another machine, transfer via USB. In Safe Mode, run DDU, select GPU → Clean and restart. This wipes all GPU drivers, registry entries, and leftover files. Don't skip this — uninstalling through Device Manager leaves junk.
Install the correct driver for your primary GPU. After reboot, Windows will install a basic VGA driver. Download the exact driver for your primary GPU from NVIDIA, AMD, or Intel. Install it normally. Reboot.
Verify the error is gone. Open Device Manager, check that the GPU shows under Display adapters with no yellow bang. Run dxdiag or a game to confirm.
Power down and install the second GPU. Only now should you add the secondary card. Boot normally — Windows will detect it and prompt for drivers. Let it install from Windows Update or manually grab the drivers.

If the main fix doesn't work

Sometimes the issue is more stubborn. Here's what else can help:

Clear CMOS. If you changed UEFI settings (resizable BAR, primary display adapter, CSM), reset BIOS to defaults. Some boards cache the POST device in NVRAM. Pull the CMOS battery for 30 seconds.
Force PCIe generation. In BIOS, set PCIe Gen3 explicitly instead of Auto. I've seen Gen4 cards fail to negotiate on older boards, causing the POST adapter to be misidentified.
Use the other PCIe slot. If the primary x16 slot has a bent pin or dodgy contact, swap cards. The motherboard's POST logic might still prefer slot 0, but at least the driver can match.
Disable the onboard iGPU. If you're using a discrete GPU and the CPU has integrated graphics, disable it in BIOS. Windows can get confused about which device posted. Set IGPU Multi-Monitor to Disabled.
Update motherboard BIOS. I've seen several Gigabyte and ASUS boards between 2020 and 2023 ship with buggy ACPI tables that misreport the POST device after a GPU swap. A BIOS update often fixes it.

Prevention tip

Before swapping or adding a GPU, always uninstall the current GPU driver completely using DDU while the old card is still installed. Shut down, swap hardware, then install the new driver. This prevents the POST device mismatch entirely because the new driver has no record of the old POST adapter. Also, don't assume Windows Update will pull the right driver — it often grabs a generic one that triggers this error on dual-GPU laptops or desktops with mixed vendors.