Troubleshooting: 502 errors, crashes and slow apps

Where to look

• App settings → Logs — your app's own output and errors
• Logs → Error log — the web server's errors for the site
• Logs → Access log — every request and its status

502 Bad Gateway

• The app is not listening on PORT — it must use process.env.PORT / $PORT and 127.0.0.1
• The start command is wrong — check it in App settings
• The app crashed on start — read App settings → Logs (missing package? missing variable?)

Crashes and restarts

• Out of memory — the plan's RAM is a hard limit for the whole account; upgrade the plan or lower memory use
• Missing environment variable — add it and the app restarts

Changes not showing

Code only: touch tmp/restart.txt. New packages: press Deploy. Behind Cloudflare: purge its cache.