HTTP 503 status code (function execution error) - Amazon CloudFront
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HTTP 503 status code (function execution error)

If you're using Lambda@Edge or CloudFront Functions, an HTTP 503 status code can indicate that your function returned an execution error.

For more information about troubleshooting Lambda@Edge errors, see Testing and debugging Lambda@Edge functions.

For more information about troubleshooting CloudFront Functions, see Testing functions.