Auto Scaling periodically performs health checks on the instances in your group and replacesinstances that fail these checks. By default, these health checks use the results of EC2 instance statuschecks to determine the health of an instance. If you use a load balancer with your Auto Scalinggroup, you can optionally choose to include the results of Elastic Load Balancing health checks. AutoScaling marks an instance unhealthy if the calls to the Amazon EC2 action DescribeInstanceStatusreturns any other state other than running, the system status shows impaired, or the calls to ElasticLoad Balancing action DescribeInstanceHealth returns OutOfService in the instance state field. Afteran instance is marked unhealthy because of an Amazon EC2 or Elastic Load Balancing health check, itis scheduled for replacement. You can customize the health check conducted by your Auto Scaling group by specifying additionalchecks or by having your own health check system and then sending the instance's healthinformation directly from your system to Auto Scaling. Reference: http://docs.aws.amazon.com/AutoScaling/latest/DeveloperGuide/healthcheck.html