An ALB will not send traffic to unhealthy EC2 instance and will auto scale by itself to replace the failing instance with a healthy one

Explanation

After determining the availability of a new EC2 instance, the load balancer starts sending traffic to it. If ELB determines that an EC2 instance is no longer working, it stops sending traffic to it and informs Amazon EC2 Auto Scaling. It is the responsibility of Amazon EC2 Auto Scaling to remove that instance from the group and replace it with a new EC2 instance. Traffic is only sent to the new instance if it passes the health check.