Health checks failed with these codes: [502]

From https://docs.aws.amazon.com/elasticloadbalancing/latest/application/load-balancer-troubleshooting.html#http-502-issues

HTTP 502: Bad gateway

Possible causes:

The load balancer received a TCP RST from the target when attempting to establish a connection.

The load balancer received an unexpected response from the target, such as "ICMP Destination unreachable (Host unreachable)", when attempting to establish a connection. Check whether traffic is allowed from the load balancer subnets to the targets on the target port.

The target closed the connection with a TCP RST or a TCP FIN while the load balancer had an outstanding request to the target. Check whether the keep-alive duration of the target is shorter than the idle timeout value of the load balancer.

The target response is malformed or contains HTTP headers that are not valid.

The target response header exceeded 32 K for the entire response header.

The deregistration delay period elapsed for a request being handled by a target that was deregistered. Increase the delay period so that lengthy operations can complete.

The target is a Lambda function and the response body exceeds 1 MB.

The target is a Lambda function that did not respond before its configured timeout was reached.

The target is a Lambda function that returned an error or the function was throttled by the Lambda service.

The load balancer encountered an SSL handshake error when connecting to a target.

Please also see https://repost.aws/knowledge-center/elb-alb-troubleshoot-502-errors