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Check the health check configuration: Verify that the health check configuration is correct, including the target group, protocol, path, and response codes. Check if the health check is set up for the correct port.

Check the security group configuration: Verify that the security group associated with the target instances allows traffic from the load balancer on the correct port.

Check the network configuration: Check if the instances are reachable from the load balancer by utilizing tools such as ping, telnet, or traceroute. Check if the instances can handle the traffic from the load balancer.

Check the instance status: Verify that the instances are running and healthy. Check the logs on the instances to see if there are any errors or problems that may be causing the health check to fail.

Check the load balancer logs: Check the access logs on the load balancer to see if there are any errors or problems that may be causing the health check to fail.

Review the load balancer settings: Review the load balancer settings such as the timeout value, interval, and response timeout, and adjust them to ensure that they are appropriate for the specific application.

Check the DNS resolution: Verify that the DNS name resolution is working correctly for the load balancer DNS name.

Check other AWS services: Check if there are any issues with other AWS services that may be causing the health check to fail, such as VPC configurations or routing issues.

Contact AWS Support: If the above steps do not resolve the issue, contact AWS Support for further assistance.