The answer is misconfigured health checks or unhealthy targets. HTTP 503 errors from an Application Load Balancer occur when the ALB cannot find any healthy registered targets to forward traffic to, even though the EC2 instances may be running. This happens because health checks are failing—either due to incorrect health check settings (like a wrong path, port, or protocol) or because the application on the instance is not responding as expected. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding of how ALB routing depends entirely on target group health status; a common trap is assuming a running instance is automatically healthy. Remember, the ALB only cares about the health check response, not the OS status. Memory tip: “503 means no healthy node—check the health check code.”
SOA-C02 Networking and Content Delivery Practice Question
This SOA-C02 practice question tests your understanding of networking and content delivery. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A company hosts a web application on EC2 instances behind an Application Load Balancer (ALB). Users report intermittent timeouts. The web server logs show HTTP 503 errors. Which configuration is MOST likely causing the issue?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The target group health checks are misconfigured or the instances are unhealthy.
HTTP 503 errors from an ALB indicate that the load balancer cannot route requests to healthy targets. Misconfigured health checks or unhealthy instances cause the ALB to mark targets as unhealthy, leading to 503 responses even if the instances are running. This is the most direct cause of intermittent timeouts when the web server logs show 503 errors.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
The ALB idle timeout is set too low.
Why it's wrong here
Incorrect idle timeout causes 504 errors, not 503.
✗
The target group is not configured with enough capacity.
Why it's wrong here
Insufficient capacity can cause 503 but health check failure is more likely.
✓
The target group health checks are misconfigured or the instances are unhealthy.
Why this is correct
Unhealthy targets cause ALB to stop routing traffic, resulting in 503.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The security group for the ALB does not allow inbound traffic on port 80.
Why it's wrong here
Inbound security group rules affect connectivity, not 503 specifically.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse HTTP 503 errors with capacity issues (Option B) or timeout settings (Option A), but the specific presence of 503 errors in web server logs points directly to health check failures or unhealthy targets, not general capacity or idle timeout misconfigurations.
Detailed technical explanation
How to think about this question
The ALB performs health checks by sending requests (e.g., HTTP GET to a configured path) to each target in the target group. If the health check fails (e.g., wrong path, incorrect response code, or timeout), the target is marked unhealthy and removed from rotation. The ALB then returns 503 for any requests that would have been routed to that target. Intermittent 503s can occur if health checks are flapping due to misconfigured intervals, thresholds, or response timeouts, or if instances are intermittently failing health checks due to resource exhaustion or application errors.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Networking and Content Delivery — This question tests Networking and Content Delivery — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The target group health checks are misconfigured or the instances are unhealthy. — HTTP 503 errors from an ALB indicate that the load balancer cannot route requests to healthy targets. Misconfigured health checks or unhealthy instances cause the ALB to mark targets as unhealthy, leading to 503 responses even if the instances are running. This is the most direct cause of intermittent timeouts when the web server logs show 503 errors.
What should I do if I get this SOA-C02 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company hosts a web application on EC2 instances behind an Application Load Balancer. Users report intermittent 503 errors. Which step should the SysOps administrator take to troubleshoot the issue?
easy
✓ A.Verify the target group health check settings.
B.Enable cross-zone load balancing.
C.Increase the idle timeout on the load balancer.
D.Add more subnets to the load balancer.
Why A: The 503 error indicates the load balancer cannot connect to the targets. Checking the target group health check configuration is the first step to identify if targets are marked unhealthy.
Last reviewed: Jun 24, 2026
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This SOA-C02 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the SOA-C02 exam.
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