Common HTTP Error status codes
These are the HTTP error status codes you are most likely to see in the results of a load test, with what each one means in RFC 9110. 4xx codes usually point at the request (the URL, headers, authentication or test data); 5xx codes point at the server or something in front of it, and often appear only once load grows.
Client errors (400-499)
400 Bad Request: the server could not understand the request, for example because of malformed syntax or invalid input.
401 Unauthorized: the request lacks valid authentication credentials for the resource.
403 Forbidden: the server understood the request but refuses to fulfil it. Authenticating again will not change the answer.
404 Not Found: the server has no resource at that URL. In a load test it is usually a wrong URL or path.
405 Method Not Allowed: the resource does not support the HTTP method you used (for example POST on a read-only URL).
408 Request Timeout: the server did not receive a complete request within the time it was prepared to wait and is closing the connection.
429 Too Many Requests (RFC 6585): the client sent too many requests in a given time. It is how rate limiting is usually reported, so it is common in load tests.
Server errors (500-599)
500 Internal Server Error: an unexpected condition stopped the server from fulfilling the request.
501 Not Implemented: the server does not support the functionality the request needs, for example it does not recognise the method.
502 Bad Gateway: a gateway or proxy received an invalid response from the upstream server.
503 Service Unavailable: the server cannot handle the request right now, typically because it is overloaded or down for maintenance.
504 Gateway Timeout: a gateway or proxy did not get a response from the upstream server in time.
507 Insufficient Storage (WebDAV, RFC 4918): the server cannot store what is needed to complete the request.
To see how these appear in your results, read Errors by Type and Error Rate. For troubleshooting steps per status class, see Errors and Response Codes.