n8n: Resolving 403 Forbidden Error Converting Google Slides to PDF via Google Drive Node

Below are some steps and checks you can perform in your n8n workflow and in your Google Cloud configuration:

1. Verify External URL Configuration

If you’re self-hosted, ensure that your external URLs match what’s configured on the Google Cloud Project. In particular, confirm that the environment variables used by n8n (like N8N_EDITOR_BASE_URL and WEBHOOK_URL) are set to fully qualified domains. A mismatch here can cause OAuth-related issues—even if you can authenticate, export operations (such as file conversion) might fail.

2. Check Credential Type Settings

Even though you’ve re-authorized your credentials, make sure that if you’re using a Service Account you have enabled any required options—such as impersonating a user. For Service Accounts, the “Forbidden – perhaps check your credentials” error often indicates that the Service Account isn’t properly configured with “Impersonate a User.” If you’re using OAuth2 credentials instead, double-check the allowed redirect URIs in your Google Cloud Project.

3. Validate API and OAuth Scopes

Although you already added the commonly needed scopes:

• https://www.googleapis.com/auth/drive
• https://www.googleapis.com/auth/drive.file
• https://www.googleapis.com/auth/presentations.readonly

It may help to verify in the Google Cloud Console that the APIs you’re using (both Drive and Slides APIs) are enabled for your project. Also, make sure that the OAuth consent screen is fully set up even if the app isn’t “verified.” In some cases, Google may restrict certain operations if the app wasn’t verified even when you’re the owner.

4. Confirm Google Slides Export Support

Downloading a Google Slides presentation as a PDF uses Google Drive’s export functionality. Although the file’s permissions seem correct, it’s useful to test whether a simple export (outside of conversion) via a separate tool or using a direct API call works as expected. This can help narrow down whether the issue is specific to n8n’s handling of file conversion.

5. Debug with a Basic Get Operation

Try using a Google Drive node configured to simply “Get” the file (using the file’s ID) without any conversion. If that operation succeeds, the issue might be tied to the export conversion logic. In that case, you may need to revisit how the conversion parameters are passed in the node configuration.

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By checking these areas, you should be able to identify whether the issue is related to OAuth/credential configuration, external URL mismatches, or the conversion operation itself. If none of these steps resolve the issue, consider checking the logs for any additional details that can point to a more specific configuration or API mismatch.