OAuth2 Connection Shows as Connected But Fails to Retrieve Access Token and Data

Below are several troubleshooting steps to help resolve this issue:

1. Verify API & Scopes

• API Enabling:
  Ensure that you’ve enabled the required Google API (for example, the Gmail API if you're connecting to Gmail) in your Google Cloud Console under APIs & Services > Library. When the API isn’t enabled, Google may reject the token request even though the connection seems to be set up.

• Scopes:
  Double-check that you’ve added only the supported scopes for your use case. Unsupported or misspelled scopes can cause Google to deny the access token request. For example:

  https://www.googleapis.com/auth/gmail.labels https://www.googleapis.com/auth/gmail.compose
  

  Refer to the scopes documentation in your n8n integration guides.

2. Check OAuth Consent Screen & Redirect URI

• OAuth Consent Screen:
  Confirm that the OAuth consent screen in your Google Cloud project is correctly configured with the required details (App name, user support email, and developer contact information). An incomplete configuration may lead to issues when fetching the token.

• Redirect URI:
  Verify that you have correctly set the n8n OAuth Redirect URL in your Google Cloud OAuth client credentials. Even a small mismatch here can result in errors where Google accepts the connection but fails to retrieve the token.

3. Review Credential Setup

• Credentials Consistency:
  Make sure that the Client ID and Client Secret entered into your n8n credential are copied exactly as provided by Google. Any discrepancies could lead to authentication issues even when the connection is reported as successful.

4. Error Message Insight

• Unauthorized Client Errors:
  The error message you see in Google Cloud (e.g., “unauthorized_client”) often signifies that Google is rejecting the token request due to issues like incorrect redirect URIs, unsupported scopes, or misconfigured OAuth consent screens. Revisiting the OAuth setup steps in either the Google OAuth2 generic or Google OAuth2 single service documentation can be very helpful.

Summary

To achieve a successful OAuth2 flow that retrieves an access token:

• Verify that all required APIs and scopes are correctly enabled.
• Ensure that the OAuth consent screen is fully configured.
• Double-check that your n8n credential’s Client ID, Client Secret, and Redirect URI perfectly match the settings in the Google Cloud Console.

Following these steps should help resolve the issue where the credentials appear connected but no access token is fetched. If you continue to experience problems, additional details about your setup (such as whether you’re using OAuth2 generic or single service) might be necessary for further troubleshooting.