HTTP 503 is the correct response that will appear in the Application Load Balancer logs when the listener does not have a target group attached or when the target group has no registered targets.

HTTP 503 is returned because the load balancer cannot forward the request to any backend. The ALB is able to accept the request but it has no backend endpoints to route to, so it replies with Service Unavailable rather than timing out or denying access.

HTTP 500 denotes an internal server error in the load balancer or backend processing and it is not the expected response for an unattached or empty target group.

HTTP 504 indicates a gateway timeout that occurs when the load balancer reached a target but did not receive a response before the timeout, and this situation does not apply when there are no targets attached.

HTTP 403 means the request is forbidden and is usually caused by access controls such as AWS WAF rules or listener rules, which is unrelated to a missing target group assignment.

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