How do you troubleshoot a 502 Bad Gateway error in NGINX?

A 502 means NGINX successfully received a request but got an invalid or no response from the upstream server it tried to proxy to - the problem is almost always on the backend side, not in NGINX itself.

• Check whether the backend application is actually running, via its process status or listening port.
• Check NGINX's error log, typically at /var/log/nginx/error.log, for the specific reason - connection refused, timeout, or reset.
• Verify the address and port in proxy_pass match where the backend is actually listening.
• If the backend is slow, check whether it's exceeding proxy_read_timeout or similar timeout directives.
• For containerized backends, confirm networking between the NGINX container/host and the backend container is actually reachable.

Because the root cause sits behind NGINX, fixing a 502 usually means restarting, debugging, or scaling the backend application rather than changing NGINX configuration - unless timeouts or the proxied address are misconfigured.