The Mystery of 3/3 Checks on EC2—Solved

Most engineers who’ve worked with AWS Cloud will have launched an EC2 instance (a virtual machine). But only a few of them really know what the “3/3 checks passed” message on the console actually means.

This post explains what these status checks are, why they matter, and what you should do if one of them fails. This is also something that might come up in an cloud interview question 👀.

When an instance is launched or started, AWS runs three types of status checks:

1. System Status Check
2. Instance Status Check
3. Instance Reachability Check Let’s go through each in detail.

1. System Status Check

✅ What it checks: The AWS infrastructure hosting your instance — hardware, networking, and power.
⚠️ If it fails: It’s AWS’s problem. This status check is most unlikely to fail among the three.

1. Stop & start your instance (it will migrate to a new host).
2. If the issue persists, you should probably speak to aws support team through raising a support ticket.

2. Instance Status Check

✅ What it checks: Your instance’s operating system( kernel, boot process, responsiveness).
⚠️ If it fails: That’s your responsibility.

1. Check system logs in the EC2 console.
2. Use EC2 Serial Console or SSM Session Manager if SSH/RDP isn’t working.
3. Fix issues like kernel panics, misconfigured filesystems, or firewall lockouts.
4. Reboot after fixing.

3. Reachability Check

✅ What it checks: Whether the instance is accessible over the network (SSH, RDP, or service ports).
⚠️ If it fails: Likely a config issue, it’s again our responsibility.

1. Check security group rules and NACLs(basically you will have to check whether you have created the instance in right subnet with right routes)
2. Confirm route tables & internet gateway for public subnets.
3. Verify the instance firewall (ufw, iptables, Windows Firewall).

Think of it this way - if your EC2 instance passes all three checks, it’s like your VM just cleared a cloud-level health checkup. Green lights all the way means you can focus on building, not babysitting.

Until next time, Saiyonara. 👋