The codes that matter most for website monitoring
The page has moved to a new URL permanently. Browsers and search engines update their records.
The page temporarily lives elsewhere. Browsers follow it, but search engines keep the original URL.
Access requires authentication. The server doesn't know who you are yet.
The server knows who you are but won't let you in. Access is explicitly denied.
The page doesn't exist at this URL. Could be deleted, moved, or mistyped.
The client has sent too many requests in too little time. The server is rate limiting.
Something broke on the server. A catch-all for unhandled errors — the server doesn't know what else to say.
A gateway or proxy received an invalid response from an upstream server.
The server is temporarily unable to handle the request — overloaded or down for maintenance.
The gateway didn't get a response from the upstream server in time. Something upstream is too slow.