See past load balancer target health check failure reason

Hello.

You can check the number of target instances that failed the health check by looking at things like "UnHealthyHostCount," but I don't think you can check the cause of the failure.
Therefore, I think it is difficult to identify the cause of past health check failures from CloudWatch metrics.
https://docs.aws.amazon.com/elasticloadbalancing/latest/network/load-balancer-cloudwatch-metrics.html