Blog/Explainers

What HTTP status codes mean when a monitor reports one

3 min readpublished 2026-08-16

01The short answer#

Every web response starts with a three-digit status code, and the first digit is the one that matters: 2xx worked, 3xx means go somewhere else, 4xx means the request was wrong, and 5xx means the server failed.

So when an alert arrives with a code, you already know where to look. A 5xx is yours to fix. A 4xx is usually a broken link, a deleted page or a login wall.

An incident titled Checkout returning 502: a timeline from the deploy to the recovery seven minutes later, beside the log lines from that window — An incident page: the deploy, the failed checks and the recovery on one timeline, with the log lines from those minutes. Sample data.

02The ones you will actually see#

Code	Means	Usually
200	OK	All good
301, 302	Moved	A redirect, fine unless it loops
401, 403	Not allowed	A login wall or a firewall
404	Not found	A deleted page or a bad link
429	Too many requests	Rate limiting
500	Server error	A crash in your code
502, 504	Bad gateway, gateway timeout	The app behind your proxy is down or slow
503	Unavailable	Overloaded, or in maintenance

03How UpControl reads them#

An UpControl incident puts the code in its title, like "Checkout returning 502", so you know what broke before you open anything. It opens after 3 failed checks in a row and closes on the first success, so one network blip never wakes you up.

A 401, 403 or 429 gets special treatment. It usually means a firewall or bot protection blocked our probe, not that your site is down, so UpControl records "couldn't measure" instead of an outage. Your uptime counts only what was actually measured.