4xx status codes are the group of HTTP status codes that represent client-side error responses. When a user tries to access a web page through a browser, the browser sends a request to the relevant server. The server responds to this request with a three-digit HTTP status code. If the request is processed successfully, 2xx codes are usually returned. If redirection is needed, 3xx codes appear. If there is a client-side issue, 4xx codes are returned. If there is a server-side error, 5xx codes are shown.
4xx codes do not mean that the server gives no response at all. On the contrary, the server receives the request and indicates that the request cannot be fulfilled in its current form. The issue may be caused by an incorrect URL, missing authorization, invalid request, missing resource, access restriction or too many requests. For this reason, 4xx status codes are also known as client error responses.
HTTP status codes consist of three digits, and the first digit shows which class the code belongs to. Codes beginning with 4xx indicate that there is a problem related to the requesting side. This side is often seen as the user’s browser, but the error is not always directly caused by the user’s behaviour. Misconfigured links, deleted pages, faulty redirects, missing access permissions or broken site architecture can also cause 4xx errors.
One of the most well-known 4xx codes is 404 Not Found. A 404 code indicates that the requested resource could not be found on the server. The user may have clicked an incorrectly written URL, the page may have been deleted, the URL structure may have changed or there may be a broken internal link on the site. A 404 error is one of the most common status codes on the web. Isolated and natural 404 errors do not always create a major problem, but if they appear on critical pages or across many URLs, they should be reviewed from a technical SEO perspective.
410 Gone indicates that the requested resource is no longer available on the server and that this condition is permanent. With a 404 code, it is not clear whether the resource is temporarily or permanently unavailable. A 410 code gives a more definite message and indicates that the page will not return. For this reason, 410 can be used for pages that have been completely removed and are not planned to come back. However, the decision to use 410 should be made carefully for pages that have organic traffic, backlinks or user value.
403 Forbidden indicates that the server understood the request but refuses to allow access. The user may not have permission to view the page, an IP block may be active, file permissions may be incorrect or security rules may be blocking access. For example, an admin panel that is open only to authorized users may return a 403 response to external access attempts. This code indicates that the resource may exist, but access is forbidden.
401 Unauthorized is also related to access, but unlike 403, it requires authentication. A 401 response can occur when a user tries to access a protected resource without providing a valid username, password, token or session credential. With a 403 code, the user may be authenticated but still not authorized. With a 401 code, the main issue is that the request has not been made with valid authentication credentials.
400 Bad Request indicates that the server cannot process the request because it is malformed, incomplete or invalid. Incorrectly formatted URLs, broken request parameters, invalid header information or faulty data submissions can cause this code. 400 errors can appear especially on form submissions, API requests or websites with complex filtering structures. These errors can directly affect user experience and technical functionality.
429 Too Many Requests indicates that the user or a bot has sent too many requests within a specific period. This code is usually used for rate limiting, security controls, API usage limits or bot traffic management. For example, if too many requests are sent to an API in a short time, a 429 response may be returned. If this code is configured incorrectly on websites, it can also negatively affect search engine bots during crawling.
From an SEO perspective, 4xx status codes should be monitored carefully. If search engine bots repeatedly encounter 404, 403 or other 4xx codes on important pages, crawling, indexing and ranking performance may be negatively affected. Pages that receive organic traffic, have backlinks, generate conversions or play an important role in site architecture should be fixed quickly if they return 4xx errors. However, it may be normal for unnecessary, old or truly removed pages to return 404 or 410 in some cases.
The cause of 4xx errors should be analysed correctly. If a page has been permanently removed and there is no suitable replacement, 404 or 410 may be correct. However, if there is a new equivalent page, a 301 redirect may be healthier. For example, if an old product category has been replaced by a new one, users and search engines should be redirected to the relevant new page. On the other hand, redirecting every 404 to the homepage is not a good practice; it can weaken user experience and cause soft 404 issues.
Different tools can be used to detect 4xx status codes. Google Search Console, site crawling tools, server logs, CDN reports, uptime monitoring systems and technical SEO tools can all be useful in this process. URLs reported in Search Console as “Not Found,” “Unauthorized request” or “Access denied” should be reviewed regularly. However, attention should not be limited only to reported errors; the overall health of internal links, sitemap files and redirect rules should also be checked.
To reduce 4xx errors, internal links should be checked regularly. Links pointing to deleted or changed pages should be updated, sitemaps should include only accessible and indexable URLs, and important pages should not be accidentally blocked. In addition, when users arrive at an incorrect URL, offering a clear, helpful and guiding 404 page can improve user experience.
In summary, 4xx status codes are HTTP response codes indicating that the server cannot fulfill the request because of a client-side issue. Different types such as 400, 401, 403, 404, 410 and 429 each represent a different problem. Correctly interpreting these codes is important for SEO, user experience and technical site health. Not every 4xx error is equally critical; what matters is the cause of the error, the page where it appears, whether it is permanent and how it affects users or search engines.
