Start with the class
The first digit gives the broad outcome: 2xx means success, 3xx redirects or further action, 4xx indicates a client-side problem, and 5xx indicates a server failure. A response with 404 is still a completed HTTP exchange. A network error, by contrast, may have no HTTP status at all.
Read the exact code
401 generally means authentication is required or invalid; 403 means access is refused. 404 says the resource was not found, while 405 says the method is not allowed and should include an Allow header. 429 signals rate limiting and may include Retry-After.
Debug with context
Check the response headers and body, not only the status number. A 200 response can contain an application-level error, while a 204 correctly has no body. Redirects may change the final URL. If the browser reports a CORS failure, it may hide the underlying HTTP response, so compare the browser request with the API’s CORS policy.