The answer is a health check timeout set too low, which causes the ALB to mark instances unhealthy and return 503 errors. When an application is running but takes longer than the configured timeout—in this case, over 30 seconds—to respond to a health check ping, the load balancer considers the target unhealthy and stops routing traffic to it, resulting in intermittent 503 errors for users. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this question tests your understanding of how ALB health check parameters directly impact target group availability, a common troubleshooting scenario for production applications. A frequent trap is assuming security groups or scaling policies are at fault, but the key clue is that the application logs show it is running and responding slowly, not failing outright. Remember the memory tip: "Timeout = Toss-out" — if the health check timeout is too short, the ALB tosses out healthy but slow instances, causing 503s.
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.
An application running on Amazon EC2 instances behind an Application Load Balancer (ALB) is experiencing intermittent 503 errors. The target group health checks are failing. The DevOps engineer checks the instance logs and finds that the application is running but taking longer than 30 seconds to respond. What is the MOST likely cause?
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 health check timeout is set too low, causing the ALB to mark instances unhealthy.
Option B is correct because ALB health checks have a configurable timeout; if the response takes longer than the timeout, the health check fails, causing 503 errors. Option A is wrong because security groups typically do not cause intermittent failures. Option C is wrong because the application is running, so it is not crashing. Option D is wrong because scaling policies do not directly cause 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 Auto Scaling group's scaling policy is too aggressive, causing frequent instance replacements.
Why it's wrong here
Scaling policies do not directly cause 503 errors.
✗
The security group for the ALB does not allow inbound traffic from the internet.
Why it's wrong here
Intermittent errors would not occur; traffic would be consistently blocked.
✓
The health check timeout is set too low, causing the ALB to mark instances unhealthy.
Why this is correct
If the application responds slowly, health checks may timeout.
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 EC2 instances are running out of memory and the application is crashing.
Why it's wrong here
The application is still responding, albeit slowly.
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 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 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: The health check timeout is set too low, causing the ALB to mark instances unhealthy. — Option B is correct because ALB health checks have a configurable timeout; if the response takes longer than the timeout, the health check fails, causing 503 errors. Option A is wrong because security groups typically do not cause intermittent failures. Option C is wrong because the application is running, so it is not crashing. Option D is wrong because scaling policies do not directly cause 503 errors.
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.
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
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 web application on Amazon EC2 instances behind an Application Load Balancer (ALB). The application experiences intermittent 503 errors. The engineer suspects the ALB is returning these errors because the target instances are unhealthy. Which metric should the engineer monitor to confirm this suspicion?
hard
A.RequestCount
B.UnhealthyHostCount
✓ C.HealthyHostCount
D.TargetResponseTime
Why C: Option A is correct because the ALB publishes a metric 'HealthyHostCount' that shows the number of healthy targets. If this metric drops to zero, the ALB returns 503 errors. Option B is wrong because 'UnhealthyHostCount' is not a standard ALB metric. Option C is wrong because 'RequestCount' does not indicate health. Option D is wrong because 'TargetResponseTime' does not indicate health status.
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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