Fixing a 502 Bad Gateway error

A 502 Bad Gateway error means the web server (Nginx/Apache) is working, but the upstream application server (PHP-FPM, Node.js, etc.) failed to respond or crashed. The web server cannot forward your request properly.

Common causes

• PHP-FPM crashed or stopped - The PHP processor is not running
• Resource limits exceeded - Memory or CPU exhausted, process killed
• Upstream timeout - Application took too long to respond
• Application error - PHP fatal error preventing startup
• Too many connections - All available processes are busy
• Database connection failure - Cannot connect to MySQL/database

How to diagnose

Check the error logs to see what caused the failure:

# For Nginx + PHP-FPM
tail -f /var/log/nginx/error.log
tail -f /var/log/php-fpm.log

# For Apache
tail -f /var/log/apache2/error.log

# Application-specific logs (WordPress)
tail -f /var/www/yourdomain.com/wp-content/debug.log

Common log messages:

• connect() failed (111: Connection refused) - Application not listening on socket
• upstream timed out (110: Connection timed out) - Application too slow
• PHP Fatal error - PHP code crashed during startup

How to fix it

Step 1: Restart PHP-FPM

systemctl restart php-fpm

Step 2: Check PHP error logs

tail -50 /var/log/php-fpm.log | grep -i error

Step 3: Increase PHP-FPM worker processes

If traffic is high, increase available workers in /etc/php-fpm.d/www.conf:

pm.max_children = 50          # Increase from default 20
pm.start_servers = 10
pm.min_spare_servers = 5
pm.max_spare_servers = 15

Step 4: Increase timeout values

# In Nginx config
fastcgi_connect_timeout 30s;
fastcgi_send_timeout 30s;
fastcgi_read_timeout 30s;

Step 5: Check database connectivity

If using WordPress/MySQL, verify database credentials in wp-config.php are correct and MySQL is running:

systemctl status mysql
mysql -u user -p database_name -e "SELECT 1;"

Prevention tips

• Monitor PHP processes - Use htop to watch PHP-FPM worker usage
• Set appropriate limits - Allocate enough workers for your traffic
• Enable opcache - Speeds up PHP execution, reduces crashes
• Monitor resources - Keep RAM and CPU usage reasonable
• Regular backups - In case application files are corrupted

Related: Fixing a 503 Service Unavailable error | Fixing a 504 Gateway Timeout error | Error logs in cPanel

Related articles