Error code 1001 in Outlook 2016: how to fix?

Outlook 2016 refuses to launch after a failed Office update, showing error 1001. Repairing from the setup exe also dies with the same code.

Word and Excel still open fine. What does 1001 mean and how do I dig out of it?

Error 1001 is a Windows Installer custom action failure. In plain terms the installer ran a step that crashed, usually because the previous failed update left the Office installation records half written. That is why the repair fails the same way, it trips over the same broken record.

1

Try the repair from the right place first: Control Panel, Programs and Features, select Microsoft Office, Change, then Online Repair rather than Quick Repair. Online Repair rebuilds from downloaded files instead of trusting local ones.

2

If Online Repair also fails, run the Microsoft Support and Recovery Assistant (SaRA) and pick the Office removal scenario. It scrubs the broken installer records that a normal uninstall leaves behind.

3

Reinstall Office from your account portal at office.com after the scrub.

4

Your mail data survives all of this. PST and OST files live in your user profile, not the Office installation, and reconnect when Outlook signs back in.

Before the full scrub it is worth two minutes checking Windows Installer itself: run services.msc, find Windows Installer, make sure it is not disabled. A disabled installer service produces 1001 on every install action and is a common leftover from aggressive optimization guides.

Online Repair failed but SaRA plus reinstall fixed it in about 40 minutes. All mail reconnected on sign in like you said.