EC2 scale from auto scaling group not start ecs agent

The issue you're experiencing with EC2 instances from your Auto Scaling group not starting the ECS agent could be due to several reasons:

1. IAM Role Configuration: Ensure that the EC2 instances have the correct IAM role attached with the necessary permissions to communicate with ECS. The role should include policies like "AmazonEC2ContainerServiceforEC2Role".

2. Security Groups and Network ACLs: Check if the security groups and network ACLs in your VPC are configured correctly to allow the EC2 instance to communicate with ECS service endpoints.

3. ECS Agent Configuration: Verify that the ECS agent is properly configured on the instances. Check the "/etc/ecs/ecs.config" file for correct cluster name and other necessary settings.

4. VPC Route Tables: Incorrect VPC route table configurations can prevent the ECS agent from functioning properly. Ensure your route tables are set up correctly.

5. Instance Metadata Access: Make sure the EC2 instances can access the instance metadata service, which is crucial for the ECS agent to function.

6. DNS Resolution: Verify that DNS resolution is working correctly in your VPC.

To troubleshoot:

1. Check the full ECS agent logs (usually at "/var/log/ecs/ecs-agent.log") for detailed error messages.

2. Verify network connectivity between the EC2 instances and ECS service endpoints.

3. Compare configurations (IAM roles, security groups, etc.) between working and non-working instances.

4. Try manually stopping and restarting the ECS agent on affected instances.

If you can't connect via SSM, it might indicate broader networking or permission issues. Double-check the instance's network configuration and IAM roles to ensure they have the necessary permissions for both ECS and Systems Manager.

If the problem persists, you may need to terminate the problematic instances and launch new ones with the latest ECS-optimized AMI, ensuring all configurations are correct from the start.
Sources
EC2 instance does not show up at ECS container instance | AWS re:Post
AWS EC2 Azure DevOps Agent Not Starting Automatically in Auto Scaling Group | AWS re:Post

Even though you have 1 EC2 ok, Intrestingly your main issue seems to be in Zone 1a. Double check your Security Groups, NACLs, Subnets, Private Endpoints etc.