Authenticator codes not working?

It's almost always the phone's clock. Authenticator codes are calculated from the time, so a clock that's a minute off makes codes the service rejects. Set date and time to automatic, wait for a fresh code, and try again.

Fix the clock

• iPhone: Settings › General › Date & Time, and turn on Set Automatically. If it's already on, turn it off and on again.
• Android: Settings › System › Date & time, and turn on automatic date and time and automatic time zone. The menus differ a little between phone makers.

The time zone must be right too. Codes are calculated from universal time (UTC), so a wrong time zone with the clock set by hand to look right still gives wrong codes.

Google Authenticator's time sync

Google Authenticator for Android used to have its own setting for this, Time correction for codes › Sync now. Many guides still describe it, but it's gone since version 7.0: the app now uses the phone's own clock. So the fix is the phone's clock, as above. Google Account Help

Other causes

• The code changed while you typed. Each code lasts 30 seconds. Wait for a fresh one and enter it right away.
• The wrong account. With two accounts at the same service, check the name under the code.
• Two-factor login was set up again. Each new QR code from a service carries a new key. If you, or your IT department, reset two-factor login, the old entry in your app keeps showing codes that no longer work. Delete it and scan the new QR code.
• The service wants a different code. Some sign-ins ask for a code sent by text message or email, not the one in your app. Read the prompt.
• The key was typed in by hand. Choose time-based codes, not counter-based, and check for typos – a single wrong character gives codes that look fine but never work. Scanning the QR code avoids both.

Still locked out

Sign in with one of the backup codes you got when you turned on two-factor login, or use the service's account recovery. Once you're in, set up two-factor login again and scan the new QR code. For your Google Account: backup codes.

Kode

Kode is an iPhone app by Peter Theill ($1.99 once) that makes the same six-digit codes, with no account and no ads, and your keys stay in your own Keychain. Like every authenticator app, it calculates codes from the iPhone's clock – so if a code doesn't work, the clock is the first thing to check there too.