The answer is to investigate health check configuration and target group health status, insufficient instances in the Auto Scaling group, and the ALB’s idle timeout settings. When the ALB’s health checks fail or target instances are marked unhealthy, the load balancer has no healthy targets to route traffic to, resulting in a 503 error. Similarly, if the Auto Scaling group lacks enough running instances to handle the current request volume, the ALB cannot distribute the load and returns 503. The idle timeout setting can also cause 503 errors if it is set too low, forcing premature closure of connections before the application finishes processing. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this question tests your understanding of ALB error codes and their root causes, often using 503 as a trap to distinguish from SSL-related 502 errors or security group issues. Remember the mnemonic “HIT” for Health, Instances, and Timeout to quickly recall the three common 503 triggers.
DOP-C02 Incident and Event Response Practice Question
This DOP-C02 practice question tests your understanding of incident and event response. 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 uses an Application Load Balancer (ALB) in front of an Auto Scaling group of EC2 instances. The application is experiencing intermittent HTTP 503 errors. The DevOps team needs to diagnose the cause. Which THREE of the following should the team investigate? (Choose THREE.)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Auto Scaling group minimum capacity and scaling policy
Option A is correct because if the instances are unhealthy, the ALB returns 503. Option C is correct because insufficient instances cannot handle the load. Option E is correct because idle timeout can cause premature connection closure leading to 503. Option B is wrong because SSL certificate issues cause 502 or 400 errors. Option D is wrong because security group inbound rules affect access, but 503 is from ALB, not the client.
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.
✗
Security group inbound rules for the ALB
Why it's wrong here
Inbound rules affect client access, but 503 is from ALB.
Auto Scaling group minimum capacity and scaling policy
Why this is correct
Not enough instances can cause 503.
Related concept
Read the scenario before looking for a memorised answer.
✓
ALB idle timeout settings
Why this is correct
Timeout can cause connection drops leading to 503.
Related concept
Read the scenario before looking for a memorised answer.
✓
Health check configuration and target group health status
Why this is correct
Unhealthy targets cause 503.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
Got this wrong? Here's your next step.
Identify which DOP-C02 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Incident and Event Response — This question tests Incident and Event Response — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Auto Scaling group minimum capacity and scaling policy — Option A is correct because if the instances are unhealthy, the ALB returns 503. Option C is correct because insufficient instances cannot handle the load. Option E is correct because idle timeout can cause premature connection closure leading to 503. Option B is wrong because SSL certificate issues cause 502 or 400 errors. Option D is wrong because security group inbound rules affect access, but 503 is from ALB, not the client.
What should I do if I get this DOP-C02 question wrong?
Identify which DOP-C02 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
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 runs a multi-tier web application on EC2 instances behind an Application Load Balancer. The application experiences intermittent 503 errors during peak traffic. The Auto Scaling group is configured with a step scaling policy based on CPU utilization. CloudWatch metrics show that CPU utilization never exceeds 70%, but the ALB target group reports that some targets are unhealthy. What is the MOST likely cause?
hard
✓ A.The application health check endpoint is returning HTTP 5xx or timing out.
B.The ALB is misconfigured with an incorrect security group blocking traffic to the targets.
C.The ALB connection draining settings are too short, causing in-flight requests to fail.
D.The step scaling policy is too aggressive and is terminating instances prematurely.
Why A: Option A is correct because health checks are failing, causing the ALB to stop sending traffic to those instances, which results in 503 errors. The CPU utilization might be low because the unhealthy instances are not receiving traffic. Option B is wrong because if the scaling policy were too aggressive, you'd see more instances and possibly lower CPU, not 503 errors. Option C is wrong because connection draining does not cause health check failures. Option D is wrong because the application itself is failing health checks, not the ALB configuration.
Last reviewed: Jun 20, 2026
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This DOP-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 DOP-C02 exam.
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