See Matt.ias.be's blog post https://ma.ttias.be/nginx-and-php-fpm-upstream-timed-out-failed-110-connection-timed-out-or-reset-by-peer-while-reading/ for some tips on debugging. Can be tough sometimes though. In my Kubernetes / Docker case it was the wrong fastcig_pass in nginx config and the wrong container port. See https://imwz.io/docker-nginx-502-bad-gateway/