I deliberately stopped my Node.js backend on AWS EC2 to see what NGINX does. It returned 502 Bad Gateway.
Full lab with 4 screenshots here: https://blog.sirvickcloud9.online/2026/10/how-i-fixed-nginx-502-bad-gateway-error.html
Setup
EC2 Ubuntu + Node.js on 3000 + NGINX reverse proxy.
Browser -> NGINX -> Node.js (127.0.0.1:3000)
The Key Lesson
502 does NOT mean NGINX is down. It means NGINX cannot get a valid response from upstream.
I confirmed with:
bash
sudo nginx -t
# syntax ok - but this does NOT check backend!
sudo tail -n 20 /var/log/nginx/error.log
# connect() failed (111: Connection refused) while connecting to upstream
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