APC_INDEX_MISMATCH, the blue screen, explained and fixed

This stop code (0x00000001) is an internal Windows bookkeeping error: a thread’s asynchronous procedure call (APC) state went out of balance, which Microsoft’s documentation attributes to a driver calling kernel functions in the wrong sequence. In real life it is famous for two triggers: audio drivers, and printing. A March 2021 Windows update caused a wave of these crashes when printing to certain printers, fixed by Microsoft in an out-of-band update, which is why “it started when I printed” is a meaningful clue.

The fix, in order

  1. Note what you were doing. If the crash happens when printing, the printer driver is the suspect. If it happens with sound, video calls, or audio recording, look at audio drivers. The trigger activity narrows this one down fast.
  2. Update the printer or audio driver from the manufacturer. For printers, the maker’s current driver package, and if offered, a universal or Type 4 driver instead of an older Type 3 driver, which was implicated in the 2021 wave. For audio, the codec maker’s (often Realtek) or PC maker’s package.
  3. Check Windows Update, and uninstall a bad one if the timing matches. If the crashes started right after an update, Settings → Windows Update → Update history → Uninstall updates can remove it; then pause updates briefly and reinstall when a fixed version is offered, as Microsoft did in March 2021.
  4. Update the rest of the system drivers. Any driver can unbalance APC state; graphics, network, and chipset drivers are next on the list after the trigger-specific one.
  5. Repair system files. DISM /Online /Cleanup-Image /RestoreHealth, then sfc /scannow from an admin Command Prompt.
  6. Test in Safe Mode to confirm software. Safe Mode avoids third-party drivers almost entirely; a crash that never happens there is a driver, by elimination.

If it still happens

If printing triggers it despite current drivers, remove the printer completely (Settings → Bluetooth and devices → Printers, plus its driver in Print Server Properties) and reinstall fresh, or print through a different connection (USB instead of network) to isolate. A crash with no printing or audio pattern goes down the general driver road, then a RAM test with Windows Memory Diagnostic.

Common questions

Why does printing cause APC_INDEX_MISMATCH? Certain printer drivers, particularly older Type 3 drivers, call into the Windows graphics and print components (win32k) in ways that unbalance the kernel’s APC bookkeeping. That is why the March 2021 update that changed those components produced this exact crash at print time on specific printer models, and why updating or replacing the printer driver fixes it.

Is APC_INDEX_MISMATCH a Windows bug or a driver bug? Either can produce it, which is unusual. Microsoft documents the cause as a driver misusing kernel calls, but the 2021 incident was a Windows change exposing it. Practically: if it began after an update, suspect the update and the driver it touched; if it began after new hardware or software, suspect that driver.

Related errors

KMODE_EXCEPTION_NOT_HANDLED · SYSTEM_THREAD_EXCEPTION_NOT_HANDLED · CRITICAL_PROCESS_DIED · BAD_POOL_CALLER