How to Fix nginx 502 Bad Gateway in Under 5 Minutes

A 502 Bad Gateway error from nginx means the backend application (Node, Python, Rails, etc.) behind nginx is not responding, not that nginx itself is broken. The article provides a step-by-step diagnostic checklist: verify the app process is running, read the app logs for the real error, restart the app, confirm the nginx upstream config matches the app's port, and check for disk/RAM exhaustion. It also recommends using PM2's auto-restart configuration to prevent recurrence.