Error 503 service not reachable

The 503 Service Unavailable error you're experiencing with your EC2 instances running WordPress could be caused by several factors. Here are some suggestions to help you troubleshoot and potentially resolve the issue:

1. Check server resources: Ensure that your EC2 instances have sufficient CPU, memory, and disk space. High resource utilization can cause intermittent 503 errors.

2. Review web server configuration: Verify that your web server (likely Apache or Nginx) is configured correctly and can handle the incoming traffic. You may need to adjust settings like worker processes or connection limits.

3. Examine PHP settings: WordPress runs on PHP, so check your PHP configuration. You might need to increase PHP memory limits or adjust other PHP-related settings.

4. Investigate WordPress plugins: Disable plugins one by one to see if a particular plugin is causing the issue. Some plugins can cause performance problems or conflicts.

5. Check database connections: Ensure that your WordPress database is functioning correctly and that there are no connection issues.

6. Monitor server logs: Review your web server error logs (usually found in /var/log/apache2/error.log or /var/log/nginx/error.log) for any specific error messages that coincide with the 503 errors.

7. Consider caching: Implement a caching solution (if not already in place) to reduce the load on your servers and improve response times.

8. Load balancer configuration: If you're using a load balancer, verify that it's configured correctly and that health checks are passing for all instances.

9. Security groups and network ACLs: Confirm that your security groups and network ACLs allow incoming traffic on the necessary ports.

10. Implement monitoring and alerts: Set up CloudWatch alarms to notify you of high resource utilization or other potential issues before they lead to 503 errors.

Since refreshing the page often resolves the issue, it's possible that the problem is related to temporary resource constraints or connection limits. Implementing a more robust caching strategy and optimizing your WordPress installation could help mitigate these intermittent issues.

If the problem persists after trying these suggestions, you may need to engage with AWS support or a WordPress specialist to conduct a more in-depth investigation of your specific setup.
Sources
Website Down with 503 Error | AWS re:Post
503 Service Temporarily Unavailable with load balancer | AWS re:Post

503 Service Unavailable on Wordpress AL2023 may be PHP FPM error.

Trying checking the error logs in your /var/log/php-fpm folder.