- A
The SSL certificate on the ALB has expired.
Why wrong: An expired certificate would cause a 502 Bad Gateway error.
- B
The target group health checks are failing.
If health checks fail, the ALB stops routing traffic to those instances, causing 503 errors.
- C
The ALB DNS name is not resolving.
Why wrong: DNS resolution is client-side; the ALB itself can still process requests.
- D
The security group for the ALB is blocking traffic.
Why wrong: Security group misconfigurations would cause connection timeouts, not 503 errors.
Question 835 of 1,616
Common Causes of Intermittent 503 Errors from Application Load Balancer
This DVA-C02 practice question tests your understanding of troubleshooting and optimization. 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 EC2 instances are in an Auto Scaling group. 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 choices
Why each option matters
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 failing.
The intermittent 503 errors indicate that the ALB temporarily has no healthy targets to forward requests to. When target group health checks fail, the ALB marks instances as unhealthy and stops routing traffic to them, causing a 503 response if all instances are unhealthy. This aligns with the Auto Scaling group potentially launching new instances that haven't passed health checks yet, or existing instances failing health checks due to application overload or misconfiguration.
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 SSL certificate on the ALB has expired.
Why it's wrong here
An expired certificate would cause a 502 Bad Gateway error.
- ✓
The target group health checks are failing.
Why this is correct
If health checks fail, the ALB stops routing traffic to those instances, causing 503 errors.
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 ALB DNS name is not resolving.
Why it's wrong here
DNS resolution is client-side; the ALB itself can still process requests.
- ✗
The security group for the ALB is blocking traffic.
Why it's wrong here
Security group misconfigurations would cause connection timeouts, not 503 errors.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse 503 errors with SSL or DNS issues, but 503 specifically indicates the ALB is reachable and functioning but has no healthy targets to serve the request.
Detailed technical explanation
How to think about this question
The ALB's target group health checks are performed at a configurable interval (default 30 seconds) using HTTP/HTTPS or TCP pings. When health checks fail consecutively (default threshold: 2 failures), the instance is marked unhealthy and deregistered. The 503 error occurs when the sum of healthy instances across all target groups is zero, causing the ALB to return a '503 Service Unavailable' response. In Auto Scaling scenarios, this can happen during scale-up delays or if the application's health check endpoint (e.g., /health) is not responding correctly due to resource exhaustion.
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.
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FAQ
Questions learners often ask
What does this DVA-C02 question test?
Troubleshooting and Optimization — This question tests Troubleshooting and Optimization — 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 failing. — The intermittent 503 errors indicate that the ALB temporarily has no healthy targets to forward requests to. When target group health checks fail, the ALB marks instances as unhealthy and stops routing traffic to them, causing a 503 response if all instances are unhealthy. This aligns with the Auto Scaling group potentially launching new instances that haven't passed health checks yet, or existing instances failing health checks due to application overload or misconfiguration.
What should I do if I get this DVA-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
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 →
Same concept, more angles
2 more ways this is tested on DVA-C02
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 web application running on Amazon EC2 instances behind an Application Load Balancer (ALB) is experiencing intermittent 503 errors. Which TWO steps should be taken to diagnose the issue?
easy- A.Check the Route 53 health checks for the domain.
- ✓ B.Check the CPU utilization of the EC2 instances.
- ✓ C.Check the target group health check settings and instance health status.
- D.Check the security group rules for the ALB.
- E.Check the EBS volume type of the EC2 instances.
Why B: Option B is correct because high CPU utilization on EC2 instances can cause them to become unresponsive or fail to respond to health checks within the ALB's configured timeout, leading to 503 errors. The ALB routes traffic only to healthy targets; if instances are overwhelmed, they may fail health checks or drop requests, resulting in a 503 response to clients.
Variation 2. A company runs a Node.js application on Amazon EC2 instances behind an Application Load Balancer (ALB). Users report intermittent 503 errors. The ALB target group health checks are failing. The developer checks the EC2 instance logs and sees no application errors. What is the MOST likely cause?
hard- A.The health check path is set to '/' but the application serves on a different path.
- B.The EC2 instances are running out of memory.
- ✓ C.The health check path returns a 5xx status code due to a missing dependency.
- D.The security group for the EC2 instances does not allow inbound traffic from the ALB.
Why C: The correct answer is C. Health checks fail when the target path returns a non-2xx/3xx status code. A missing application dependency can cause the health check endpoint to return a 5xx error, while the main application may still partially function or show no errors in logs. Option A is wrong because the health check path mismatch would cause immediate failure; the issue is intermittent. Option B is wrong because out-of-memory errors would appear in logs or instance metrics. Option D is wrong because security group misconfiguration would cause constant failure, not intermittent.
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Last reviewed: Jul 4, 2026
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