5xx Server errors
501 Not Implemented
The server doesn't support what the request needs.
Common causes
- A request method the server doesn't know.
How to fix it
- Use a supported method, or implement the feature on the server.
Find out the moment your site returns 501
Spot Downtime monitors count this as down and alert you, since visitors see an error. (The public “Is it down?” checker treats 4xx answers from big sites as online, because many refuse automated requests.)