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A 502 Bad Gateway on a WordPress site running behind Nginx means Nginx could not obtain a usable response from its upstream, normally PHP-FPM. The reliable fix is to identify the exact error-log signature, verify that PHP-FPM is running and listening where Nginx expects it, then check capacity, database/cache dependencies, and the specific WordPress request. Do not start by increasing timeouts: a missing socket, crashed PHP worker, permission error, or exhausted backend will not be repaired by waiting longer.
What a 502 means in an Nginx WordPress stack
Nginx is the gateway in this arrangement. It serves static files and forwards dynamic PHP requests to PHP-FPM through either a Unix socket or a TCP listener. The Nginx fastcgi_pass value must match the PHP-FPM pool’s listen directive exactly. If Nginx cannot connect, cannot receive response headers, or receives a connection that closes unexpectedly, it returns 502 to the browser.
The browser-facing error page does not identify the failing component. BOA’s troubleshooting documentation describes 502 and 504 responses as Nginx reports that are almost always caused downstream, such as PHP-FPM, MySQL, Valkey/Redis, or the application itself. Treat the status code as a routing symptom, not a diagnosis.
Capture the failure before changing anything
- Reproduce the problem once. Record the complete URL, HTTP method, approximate UTC time, logged-in state, and whether the failure affects the whole site, only
/wp-admin, the REST API,admin-ajax.php, or one public route. - Open a second request in a private browser window or with a command-line client. This distinguishes a user-session or cache problem from a server-wide failure.
- Note whether the response is consistently 502, changes to 504, redirects repeatedly, or succeeds intermittently. Intermittent failures often indicate worker or memory pressure rather than a permanently wrong path.
- Do not reload Nginx or restart PHP-FPM before collecting at least one timestamped log entry unless the site is in an active outage and you have an approved recovery procedure. A restart can erase the evidence of a crash or queue buildup.
Read Nginx and PHP-FPM logs first
NGINX documentation calls logging essential for understanding how traffic moves through infrastructure. The error-log location depends on the installation; a common Linux path is /var/log/nginx/error.log. Use the configured path if your distribution or hosting panel stores logs elsewhere.
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sudo tail -n 100 /var/log/nginx/error.log
sudo tail -f /var/log/nginx/error.logIn a second terminal, locate the PHP-FPM service and its logs. Service names vary by distribution and installed PHP package, so list candidates rather than assuming one versioned name.
systemctl list-units --type=service | grep -i fpm
sudo systemctl status php-fpm
sudo journalctl -u php-fpm --since "15 minutes ago"If your service is versioned, substitute its actual name, such as php8.2-fpm. Correlate Nginx and PHP-FPM entries by timestamp. A PHP fatal error, worker termination, out-of-memory event, or pool-limit warning at the same moment is more useful than the generic 502 page.
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| Nginx signature | What it usually means | First action |
|---|---|---|
connect() failed (111: Connection refused) |
No process is accepting connections on the configured socket or TCP port, or a firewall/security policy is rejecting the connection. | Check PHP-FPM state, then verify the listener and service logs. |
connect() failed (2: No such file or directory) |
The Unix-socket path in fastcgi_pass does not exist. PHP-FPM may be stopped, using another path, or creating the socket in a different runtime directory. |
Compare the Nginx target with the pool’s listen value and confirm the file exists. |
upstream timed out |
The backend did not connect or return headers/body within the configured interval. Slow PHP code, a saturated pool, database latency, or a blocked external call are common causes. | Inspect PHP-FPM queue/worker limits, memory, database and cache latency before changing timeout values. |
upstream sent too big header |
The response headers exceed Nginx’s configured FastCGI buffer capacity. Large cookie sets or plugin-generated headers can trigger it. | Identify the endpoint and header source; remove unnecessary cookies/headers before increasing buffers cautiously. |
upstream prematurely closed connection |
PHP-FPM or the application closed the connection before Nginx received a complete response, often after a fatal error, crash, forced termination, or memory kill. | Check PHP-FPM, PHP, kernel, and application logs for the matching timestamp. |
Verify that PHP-FPM is running and listening
Check service health
sudo systemctl is-active php-fpm
sudo systemctl status php-fpm --no-pager
sudo journalctl -u php-fpm -b --no-pager | tail -n 100An inactive or failed service explains a connection-refused error. Read the failure reason before restarting. Configuration syntax errors, an occupied socket, a missing directory, or a permission problem should be corrected first.
After a verified correction, restart only the affected service and immediately retest the original URL:
sudo systemctl restart php-fpm
curl -I https://example.com/Replace the example hostname with your site. A restart is a recovery step, not proof that the underlying cause is fixed; if workers fail again, return to the service log.
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Find the pool’s listener
PHP-FPM pool files commonly live below /etc/php/*/fpm/pool.d/, /etc/php-fpm.d/, or a hosting-panel-specific directory. Search the active configuration for listen:
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sudo nginx -T | grep -n "fastcgi_pass|server_name"Make the two targets identical, including the full socket path, IP address, and port. Do not change one side by guesswork. A socket with a similar filename, a different PHP version, or a relative path is still a mismatch.
Confirm the socket or TCP port exists
For a Unix socket, check the file and its parent directory:
sudo ls -l /run/php/php-fpm.sock
sudo namei -l /run/php/php-fpm.sockThe socket must exist after PHP-FPM starts. Nginx’s worker user needs permission to traverse every parent directory and connect to the socket. The pool’s listen.owner, listen.group, and listen.mode settings control this on many installations. Align them with the Nginx worker account instead of making the socket world-writable.
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For a TCP listener, verify that a process is bound to the expected address and port:
sudo ss -ltnp | grep ':9000'Use the port shown in your pool configuration. If PHP-FPM listens only on 127.0.0.1, Nginx on the same host can use it; a container or separate host requires the appropriate reachable address and network policy. Keep the listener private unless there is a deliberate, secured architecture requiring otherwise.
Validate Nginx safely before reloading
After editing a server block or changing fastcgi_pass, keep a rollback copy and test the complete configuration:
sudo cp /etc/nginx/nginx.conf /etc/nginx/nginx.conf.backup-$(date +%Y%m%d-%H%M%S)
sudo nginx -tProceed only when the test reports that the syntax is successful and the configuration test is successful. Then reload rather than stopping the service:
sudo systemctl reload nginxIf nginx -t fails, do not reload. Restore the known-good file or correct the reported include, brace, path, or directive error. A failed reload leaves the existing worker configuration in place, while an unvalidated restart can turn a 502 into a complete web-server outage.
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When the error is a timeout, investigate workload and capacity
An upstream timed out entry means the request took too long at a particular stage; it does not automatically mean Nginx’s timeout is too short. Increasing fastcgi_read_timeout can merely keep more requests waiting while PHP-FPM workers, memory, or the database remain saturated.
- Check PHP-FPM pool limits such as
pm.max_children,pm.max_requests, and the selected process manager mode. A full pool leaves new requests queued or rejected. - Look for warnings that the pool has reached its maximum children, and compare that with CPU, RAM, and swap usage.
- Inspect the database for slow queries, connection exhaustion, locks, or a database service restart at the failure time.
- Check Valkey/Redis or another object-cache service if WordPress depends on it. An unavailable cache can cause slow fallback queries or application errors.
- Review scheduled tasks, backup jobs, imports, image processing, and external API calls that may consume workers.
free -h
uptime
ps -eo pid,ppid,cmd,%mem,%cpu --sort=-%mem | head
sudo journalctl -k --since "15 minutes ago" | grep -i "oom|killed process"Kernel out-of-memory messages, repeated worker exits, or a steadily growing queue indicate a capacity or application problem. Reduce the expensive workload, fix the query or plugin task, or size the pool for available memory before adjusting gateway timeouts.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Separate a site-wide failure from a WordPress endpoint failure
Every dynamic URL returns 502
Prioritize the infrastructure path: PHP-FPM state, listener agreement, socket permissions, and host memory. Test a lightweight PHP endpoint and a normal WordPress page separately. If both fail with the same connection error, changing a theme or plugin is unlikely to help.
Only the WordPress dashboard, REST API, or AJAX fails
A scoped failure points toward application code, authentication hooks, scheduled work, or a request-specific dependency. Use the exact failing path and method in your reproduction notes. Inspect PHP and WordPress logs at that timestamp, then disable or roll back the most recent plugin, theme, configuration, or scheduled task through a controlled process.
If dashboard access is unavailable, use an approved administrative route such as a deployment rollback or command-line WordPress management tool already installed on the server. Do not delete plugin directories or edit production data blindly. Change one component at a time, retest the same request, and record which change altered the result.
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Only one public URL fails
Compare that route with a simple page and examine its query volume, uploaded data, redirect behavior, and application logs. A single expensive report, search, import, or uncached page can exhaust PHP workers while the rest of the site remains healthy. Fix the request or its code path rather than raising global limits for every visitor.
Check dependencies before blaming Nginx
WordPress PHP code can fail when its database, object cache, filesystem, or external service is unavailable. Check each dependency from the same host and network context used by PHP-FPM:
- Confirm the database service is running, accepting local connections, and not reporting connection-limit or storage errors.
- Check disk space and inode availability. PHP sessions, temporary uploads, logs, and cache writes can fail when either is exhausted.
- Verify that WordPress directories and files are writable only where required, while PHP-FPM can read the application and write its designated upload/cache paths.
- Inspect DNS and outbound connectivity if the failing request calls an external API. A blocked or slow remote call can hold a PHP worker until Nginx times out.
- Check cache-service logs and credentials when object caching is enabled; a cache outage should not be mistaken for a broken FastCGI socket.
df -h
df -i
sudo systemctl status mysql
sudo systemctl status redisService names differ, and a site may use a different database or cache product. Run status commands only for components actually present in your deployment.
Use timing variables to locate the delay
Nginx can log $upstream_connect_time, $upstream_header_time, $upstream_response_time, and $request_time. Add them to a temporary access-log format if they are not already present, then correlate a failing request by timestamp.
- A zero or missing connect time can indicate that Nginx never reached the upstream, consistent with a missing socket or refused connection.
- A long connect time points to difficulty establishing the backend connection.
- A short connect time followed by a long header time suggests PHP code, a database query, or another dependency delayed the first response header.
- A long response time after headers begin indicates slow response generation or transfer.
Interpret these values with the corresponding error-log line; a timing value alone cannot identify which PHP function or query is slow.
Enable Nginx debug logging only as a controlled escalation
NGINX documents that debug logging is disabled by default and requires a build compiled with debug support. If ordinary error and application logs do not explain the failure, a qualified administrator can temporarily set:
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error_log /var/log/nginx/error.log debug;Validate with nginx -t, reload, reproduce the problem once or for a short window, and collect the relevant lines. Debug logs can grow very large and affect performance. Revert to the previous log level immediately after collection, validate again, and reload. Never leave debug logging enabled during normal traffic without a specific operational reason.
Quick Recap
Prevent the next 502
- Keep the Nginx
fastcgi_passtarget and PHP-FPM poollistenvalue under configuration management so a PHP upgrade cannot silently change one side. - Monitor PHP-FPM worker utilization, child-reached-limit warnings, memory, disk space, database latency, and cache availability.
- Retain Nginx, PHP-FPM, PHP, database, and kernel logs long enough to correlate an incident across services.
- Test plugin, theme, PHP, and server changes in a staging environment or with a rollback plan.
- Use health checks that exercise a lightweight PHP request and a representative WordPress request, not just Nginx’s static response.
- Document the service names, socket path, pool file, Nginx server block, worker user, and rollback commands for the site.
A concise recovery checklist
- Capture one reproducible failing request and its timestamp.
- Read the Nginx error log and matching PHP-FPM/application logs.
- Classify the signature as connection failure, timeout, oversized header, or premature close.
- Confirm PHP-FPM is active and its listener exists.
- Make
fastcgi_passexactly match the pool’slistentarget and verify socket permissions. - Check memory, worker limits, database, cache, disk, and external dependencies before increasing timeouts.
- If only one WordPress endpoint fails, isolate the latest plugin, theme, task, or request-specific workload.
- Run
nginx -t, keep a rollback copy, reload safely, and retest the original URL. - Use debug logging briefly only when normal logs cannot distinguish the remaining causes.
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