NGINX 502 Bad Gateway with PHP-FPM: Production Troubleshooting Guide

NGINXPHP-FPM502 Bad GatewayWordPressLinux

What this problem usually means

A 502 Bad Gateway from NGINX usually means NGINX could not get a valid response from the upstream service. With PHP sites, that upstream is often PHP-FPM, a Unix socket, or a TCP backend that is stopped, overloaded or misconfigured.

Production caution: Restarting PHP-FPM can temporarily hide the problem. If workers are exhausted, memory is low or a plugin/script is hanging, the 502 errors may return quickly.

Common symptoms

Browser shows 502 Bad Gateway
NGINX error log mentions connect() failed
PHP-FPM service is stopped or repeatedly restarting
Site fails under traffic but works after restart
WordPress admin or checkout fails intermittently

Common causes

PHP-FPM service stopped or crashed
NGINX points to the wrong PHP-FPM socket or port
Socket permissions prevent NGINX access
PHP-FPM workers are exhausted
Timeouts are too low for slow scripts
Server is out of memory or disk space

Safe first checks

These checks are intended to help identify the direction of the issue. Always adjust paths, service names and commands for your environment.

Check NGINX errors

tail -100 /var/log/nginx/error.log

Check PHP-FPM status

systemctl status php*-fpm --no-pager

Check sockets

ls -lah /run/php/ /var/run/php/ 2>/dev/null

Check memory and disk

free -m && df -h

Typical fixes

Correct the fastcgi_pass socket or TCP target
Align NGINX user permissions with PHP-FPM socket ownership
Tune PHP-FPM pool settings for available RAM
Investigate slow scripts and long-running requests
Check logs before increasing timeouts
Add monitoring for PHP-FPM worker saturation

When to get help

Get help if the system is production-facing, customer data is involved, backups are uncertain, or the issue affects revenue, security or uptime. We can review the logs, confirm the cause and quote a fixed-scope fix where appropriate.

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