You have enabled Elastic Load Balancing HTTP health checking. After looking at the AWS Management Console, you see that all instances are passing health checks. But your customers are reporting that your site is not responding. What is the reason?

Answer options:

A.The HTTP health checking system is misreporting due to latency in inter-instance metadata synchronization.
B.The health check in place is not sufficiently evaluating the application function.
C.The application is returning a positive health check too quickly for the AWS Management Console to respond.
D.Latency in DNS resolution is interfering with Amazon EC2 metadata retrieval.

Answer – B
You need to have a custom health check which will evaluate the application functionality. It`s not enough to use the normal health checks. If the application functionality does not work and doesn’t have custom health checks, the instances will still be healthy.
If you have custom health checks, you can send the information from your health checks to Auto Scaling so that Auto Scaling can use this information. For example, if you determine that an instance is not functioning as expected, you can set the health status of the instance to Unhealthy. The next time when Auto Scaling performs a health check on the instance, it will determine that the instance is unhealthy and then launch a replacement instance.
For more information on Autoscaling health checks, please refer to the below document link: from AWS
http://docs.aws.amazon.com/autoscaling/latest/userguide/healthcheck.html