Home / AWS / Fix AWS EC2 Auto Scaling Health Check Failed

AWS

Fix AWS EC2 Auto Scaling Health Check Failed

Resolve Auto Scaling Group health check failures by fixing ELB target health, adjusting check configurations, and ensuring instance startup completes.

Published: Mar 31, 20266 min readBy FixWikiHub Editorial Team

Abstract illustration for a troubleshooting knowledge base category.

Introduction

Auto Scaling Groups monitor instance health and replace unhealthy instances. When health checks fail, ASG terminates the "unhealthy" instance and launches a replacement. But if the root cause isn't fixed, the new instance also fails health checks, creating a replacement loop.

Symptoms

In the AWS Console:

bash

Instance health: Unhealthy
Last status: ELB health check failed

Via AWS CLI:

bash

$ aws autoscaling describe-auto-scaling-groups --auto-scaling-group-name my-asg \
  --query 'AutoScalingGroups[*].Instances[*].[InstanceId,HealthStatus]'
[
  ["i-abc123", "Unhealthy"]
]

Activity history shows:

bash

Terminating instance due to health check failure
Launching new instance

Common Causes

1.Application not ready - App takes longer than grace period to start
2.Health check endpoint wrong - Path, port, or protocol misconfigured
3.Security group blocking - ELB can't reach instance on health check port
4.Instance startup failure - Application crashes or fails to start
5.Network issues - Instance can't reach dependencies
6.Resource limits - Instance runs out of CPU/memory during startup
7.DNS/routing issues - Health check hostname doesn't resolve correctly

Step-by-Step Fix

1.Check logs for specific error messages
2.Verify configuration settings
3.Test network connectivity
4.Review recent changes
5.Apply corrective action
6.Verify the fix

Step 1: Check Health Check Configuration

For ALB/NLB target groups:

bash

aws elbv2 describe-target-groups --names my-target-group \
  --query 'TargetGroups[*].[HealthCheckPath,HealthCheckPort,HealthCheckIntervalSeconds,HealthCheckTimeoutSeconds,HealthyThresholdCount,UnhealthyThresholdCount]'

For Classic Load Balancers:

bash

aws elb describe-load-balancers --load-balancer-names my-elb \
  --query 'LoadBalancerDescriptions[*].HealthCheck'

Common issues: - HealthCheckPath returns 404 or 403 - HealthCheckPort doesn't match application port - HealthCheckTimeoutSeconds too short for slow responses - HealthyThresholdCount requires too many successful checks

Step 2: Check Target Health Details

bash

aws elbv2 describe-target-health --target-group-arn arn:aws:elasticloadbalancing:region:account:targetgroup/my-tg/12345

Output shows the specific failure reason:

json

{
  "Target": {
    "Id": "i-abc123",
    "Port": 80
  },
  "TargetHealth": {
    "State": "unhealthy",
    "Reason": "Target.ResponseCodeMismatch",
    "Description": "Health checks failed with this reason: Code mismatch"
  }
}

Common reason codes:

Reason	Description
`Target.ResponseCodeMismatch`	App returned unexpected HTTP code
`Target.Timeout`	Health check timed out
`Target.ConnectionFailed`	Could not establish TCP connection
`Elb.InternalError`	ELB-side issue

Step 3: Verify Security Group Rules

The instance must accept traffic from the load balancer:

```bash # Get instance security group aws ec2 describe-instances --instance-ids i-abc123 \ --query 'Reservations[*].Instances[*].SecurityGroups[*].GroupId'

# Check security group rules aws ec2 describe-security-groups --group-ids sg-12345 \ --query 'SecurityGroups[*].IpPermissions' ```

Required rule: - Allow inbound on health check port from ELB security group (or VPC CIDR)

bash

aws ec2 authorize-security-group-ingress \
  --group-id sg-instance \
  --protocol tcp \
  --port 80 \
  --source-group sg-elb

Step 4: Test Health Check Endpoint Manually

SSH into the instance and test:

```bash # Test health check endpoint curl -v http://localhost:80/health

# Check if application is listening netstat -tlnp | grep :80

# Check application logs sudo journalctl -u myapp -f ```

The endpoint should return: - HTTP 200 status code - Response within timeout - Valid response body (if configured)

Step 5: Adjust Health Check Grace Period

If the application takes time to start, increase grace period:

bash

aws autoscaling update-auto-scaling-group \
  --auto-scaling-group-name my-asg \
  --health-check-grace-period 300

Default is 300 seconds. Increase if: - Application has long startup time - Dependencies need time to connect - Database migrations run on startup

Step 6: Check Instance Logs

```bash # Get console output aws ec2 get-console-output --instance-id i-abc123 --output text

# For instances with SSM, use Session Manager aws ssm start-session --target i-abc123 ```

Check for: - Application startup errors - Missing configuration - Dependency connection failures - Permission errors

Step 7: Review ASG Activity History

bash

aws autoscaling describe-scaling-activities \
  --auto-scaling-group-name my-asg \
  --max-records 20

Look for patterns: - Consistent failures after specific timing - Specific error messages - Multiple replacement attempts

Step 8: Create Proper Health Check Endpoint

If your app doesn't have a dedicated health endpoint:

```python # FastAPI example @app.get("/health") async def health_check(): # Check database connection try: await db.execute("SELECT 1") except Exception as e: raise HTTPException(status_code=503, detail="Database unavailable")

# Check Redis connection try: await redis.ping() except Exception as e: raise HTTPException(status_code=503, detail="Redis unavailable")

return {"status": "healthy"} ```

```javascript // Express.js example app.get('/health', async (req, res) => { const checks = { database: await checkDatabase(), redis: await checkRedis(), };

const healthy = Object.values(checks).every(v => v); res.status(healthy ? 200 : 503).json(checks); }); ```

Step 9: Set Up Proper Monitoring

Create CloudWatch alarm for health check failures:

bash

aws cloudwatch put-metric-alarm \
  --alarm-name "asg-health-check-failures" \
  --metric-name UnHealthyHostCount \
  --namespace AWS/ApplicationELB \
  --dimensions Name=TargetGroup,Value=tg-12345 Name=LoadBalancer,Value=app/my-alb/12345 \
  --statistic Average \
  --period 60 \
  --threshold 1 \
  --comparison-operator GreaterThanOrEqualToThreshold \
  --evaluation-periods 3

Verification

```bash # Check instance health status aws autoscaling describe-auto-scaling-groups \ --auto-scaling-group-name my-asg \ --query 'AutoScalingGroups[*].Instances[*].[InstanceId,HealthStatus]'

# Should show "Healthy" for running instances

# Check target health aws elbv2 describe-target-health \ --target-group-arn arn:aws:elasticloadbalancing:region:account:targetgroup/my-tg/12345 ```

[Fix AWS ALB Target Unhealthy](/articles/fix-aws-alb-target-unhealthy)
[Fix AWS EC2 Instance Not Responding](/articles/fix-aws-ec2-instance-not-responding)
[Fix AWS Auto Scaling Not Triggering](/articles/fix-aws-auto-scaling-not-triggering)

[AWS troubleshooting: Fix IAM Permission Denied - Complete Tro](fix-iam-permission-denied)
[AWS cloud troubleshooting: AWS ACM Certificate Pending Validation Because the](aws-acm-certificate-pending-validation-wrong-route53-zone)
[AWS cloud troubleshooting: AWS ALB Returns 502 Because the Target Closed the ](aws-alb-502-target-closed-connection-keepalive-timeout-mismatch)
[AWS cloud troubleshooting: Fix AWS ALB CreateListener TargetGroupNotFound Err](aws-alb-createlistener-targetgroupnotfound)
[AWS cloud troubleshooting: Fix Aws Alb Lambda 502 Bad Gateway Issue in AWS](aws-alb-lambda-502-bad-gateway)

Was this guide helpful?

Related search paths

People also search for

If the symptom is close but not identical, these search paths usually surface the right neighboring fixes faster than scrolling the full archive.

AWS EC2 Auto Scaling Health Check Failed AWS EC2 Auto Scaling Health Check Failed AWS AWS EC2 Auto Scaling Health Check Failed troubleshooting AWS EC2 Auto Scaling Health Check Failed fix Resolve Auto Scaling Group health check failures by fixing ELB target health, adjusting check configurations, and ensuring instance startup completes AWS Resolve Auto Scaling Group health check failures by fixing ELB target health, adjusting check configurations, and ensuring instance startup completes

Explore Related Topics

Browse Guides from Other Categories

Discover troubleshooting guides from related categories to expand your knowledge.

FAQ

AWS Troubleshooting FAQs

Common questions about troubleshooting and preventing similar issues

How do I know if this aws-errors troubleshooting guide applies to my situation?

This guide is designed for aws-errors issues. If you're experiencing similar symptoms described in the article, follow the step-by-step instructions. Start with the most common causes and work through the diagnostic process.

Is it safe to follow these aws-errors troubleshooting steps?

Yes, all steps are designed to be safe and non-destructive. We recommend creating backups before making significant changes and testing each step before proceeding to the next.

How long does it typically take to resolve this type of aws-errors issue?

Most aws-errors issues can be resolved within 30 minutes to 2 hours, depending on the complexity and root cause. Follow the troubleshooting flow to identify and fix the problem efficiently.

How can I prevent this aws-errors issue from happening again?

Regular maintenance, monitoring, and following best practices for aws-errors configuration can help prevent recurrence. Consider implementing automated checks and alerts for early detection.

Written by

FixWikiHub Editorial Team

Our editorial team consists of experienced DevOps engineers, systems administrators, and cloud architects with hands-on experience in production environments across AWS, Azure, GCP, and on-premises infrastructure.

Every guide undergoes technical review for accuracy and is updated when software versions, commands, or best practices change.

Last updated: Mar 31, 2026

About our team

Important Notice

Disclaimer & Safety Guidelines

The troubleshooting steps in this guide are provided for educational and informational purposes. Before applying any changes to production systems:

Test in a staging environment first — Always verify commands and configurations in a non-production environment before deploying to live systems.
Create backups — Ensure you have current backups of databases, configurations, and critical files before making changes.
Understand the impact — Review how each step may affect your specific environment, dependencies, and users.
Consult official documentation — This guide supplements, but does not replace, official vendor documentation and best practices.

FixWikiHub is not responsible for any damages arising from the use of this content. See our Terms of Use for more information.

Resources

Official Documentation & Further Reading

For authoritative information, consult the official documentation for the technologies discussed in this guide. Our troubleshooting content supplements, but does not replace, vendor documentation.

AWS Documentation — Official Amazon Web Services guides and API references
Kubernetes Documentation — Official Kubernetes documentation
Nginx Documentation — Official Nginx web server documentation
Apache Documentation — Official Apache HTTP Server documentation
Docker Documentation — Official Docker container documentation

Fix AWS EC2 Auto Scaling Health Check Failed

Introduction

Symptoms

Common Causes

Step-by-Step Fix

Step 1: Check Health Check Configuration

Step 2: Check Target Health Details

Step 3: Verify Security Group Rules

Step 4: Test Health Check Endpoint Manually

Step 5: Adjust Health Check Grace Period

Step 6: Check Instance Logs

Step 7: Review ASG Activity History

Step 8: Create Proper Health Check Endpoint

Step 9: Set Up Proper Monitoring

Verification

People also search for

Browse Guides from Other Categories

WordPress

SSL

DNS

AWS Troubleshooting FAQs

FixWikiHub Editorial Team

Disclaimer & Safety Guidelines

Official Documentation & Further Reading

Fix AWS EC2 Auto Scaling Health Check Failed

Introduction

Symptoms

Common Causes

Step-by-Step Fix

Step 1: Check Health Check Configuration

Step 2: Check Target Health Details

Step 3: Verify Security Group Rules

Step 4: Test Health Check Endpoint Manually

Step 5: Adjust Health Check Grace Period

Step 6: Check Instance Logs

Step 7: Review ASG Activity History

Step 8: Create Proper Health Check Endpoint

Step 9: Set Up Proper Monitoring

Verification

Related Issues

Related Articles

People also search for

Share this guide

More AWS Troubleshooting Guides

Browse Guides from Other Categories

AWS Troubleshooting FAQs

FixWikiHub Editorial Team

Disclaimer & Safety Guidelines

Official Documentation & Further Reading