## TL;DR
0x80070643 is a generic install-phase failure, most commonly the .NET Framework update or a Microsoft Defender definition update failing, or msiexec registration breaking mid-install. Identify which KB failed in Update History first, repair .NET and the component store, then reinstall that single KB manually from the Microsoft Update Catalog.

```text
0x80070643 windows update failed to install how to fix
```

## Use this when
- An update shows Failed to install with 0x80070643 in Settings, Windows Update, Update history.
- A .NET Framework cumulative update or security update is the one that failed.
- Defender definition updates keep failing while other updates install.

## Not for this skill when
- The update never downloads. That is a delivery or WSUS problem, not an install failure.
- Every update fails with different codes. Suspect servicing stack or disk corruption instead.

## Steps
1. Open Update history and note the exact KB number and product (for example a .NET Framework cumulative update). Verify: one specific KB is identified, not just the code
2. Run the .NET Framework Repair Tool from Microsoft, then restart. Verify: the tool completes without reporting an unrepairable fault
3. In an elevated prompt run DISM /Online /Cleanup-Image /RestoreHealth, then sfc /scannow. Verify: DISM reports the store is repairable or healthy and SFC finishes
4. Download that KB as a standalone .msu from the Microsoft Update Catalog and install it directly. Verify: the standalone installer reports success instead of rolling back
5. Restart and check for updates again. Verify: Update history shows the KB as Successfully installed

## Variant phrasings
### 0x80070643 .NET Framework update failed
The .NET-shaped version of this failure, fixed by the repair tool first.
### KB5034441 install fails 0x80070643
The WinRE partition case: if the failing KB is KB5034441, resize the recovery partition instead of repairing .NET.
### Defender definition update 0x80070643
The definition-update variant, usually cleared by updating definitions from the MpCmdRun command line tool.

Compatibility: Compatibility: Windows 10 and 11, Windows Server 2016 and later; DISM and SFC commands as shown.

## Why it happens
The code is the Windows Installer and update agent catch-all for an install that started and then failed, so it carries almost no diagnostic value on its own. The common roots are a damaged .NET Framework installation the update tries to patch, a component store fault DISM can repair, or, for KB5034441, a WinRE recovery partition with under 250 MB of free space.

## Edge cases / pitfalls
- KB5034441 is the exception: 0x80070643 there means the WinRE partition is too small, and no amount of .NET repair helps.
- If only Defender definitions fail, run MpCmdRun -SignatureUpdate from the Defender platform folder before deeper repair.
- Do not reset all Windows Update components as a first step; you lose update history that names the failing KB.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_PwWso6Zps2yHLzND3Ck23w