501 Not Implemented Error: What Is It & How to Fix It

The HTTP 501 Not Implemented error belongs to the 5xx class of status codes, which signal server-side failures. Unlike the more generic 500 Internal Server Error, a 501 has a precise definition: it indicates that the server either does not recognize the HTTP request method or does not support it.

In practical terms, this happens when a client (such as a browser or API consumer) sends a request using an HTTP method - commonly POST, PUT, PATCH, or DELETE - that the server is unable or unwilling to handle. Instead of executing the request, the server replies that the functionality required is simply not implemented.

According to the HTTP/1.1 specification (RFC 7231, Section 6.6.2), the 501 status code is reserved for situations where “the server does not support the functionality required to fulfill the request.” This makes it distinct from temporary failures (like 502 or 503), because a 501 explicitly points to a missing or unsupported feature in the server’s configuration or software.

For an online store, the consequences of a 501 are not theoretical. Imagine a checkout process relying on a REST API call to update a cart via PATCH. If the hosting environment does not support that method, the server will return 501, and the customer will be unable to access the necessary resource (such as the cart or checkout page), directly affecting their ability to complete the transaction. This not only means a lost transaction but also increases the risk that the customer will abandon the website altogether.

A 501 Not Implemented error usually stems from infrastructure or configuration gaps rather than transient server overload. It indicates that the server lacks support for the HTTP method being used, or that the request was blocked before it could be processed.

Some of the most frequent are listed below.

Unsupported HTTP Methods
If the server (or its HTTP stack) does not support certain methods such as PUT, PATCH, or DELETE, it may return 501 instead of executing the request. This often happens in older or minimal builds of Apache or Nginx, where support for advanced methods is not compiled or enabled by default.

Outdated or Limited Server Software
A legacy server version may simply not recognize modern HTTP/1.1 or HTTP/2 features. For example, some older IIS installations respond with 501 when encountering unrecognized WebDAV methods.

Proxy or CDN Misconfiguration
Reverse proxies, load balancers, or CDN layers can intercept traffic incorrectly. If they fail to forward a request method they don’t support, the client sees a 501 even though the origin server could have processed it.

Application or CMS Extensions
In platforms such as Magento, WooCommerce, or other CMS-driven sites, plugins may rely on methods that the hosting environment is not set up to handle. This mismatch between application logic and server capabilities often results in a 501.

Conflicting Modules or Middleware
A server may return 501 if third-party modules, custom middleware, or security filters are configured to block certain request types.

Unlike temporary 5xx errors (e.g., 503 Service Unavailable), a 501 signals a missing feature or lack of implementation. Fixing it usually requires changes at the server or proxy level, not just adjustments in the application code.

Troubleshooting a 501 Not Implemented error involves identifying whether the issue originates from the client, server, or intermediary layers like proxies or CDNs. Below is a structured approach to resolving the problem:

Note! If a 501 error reproduces across multiple browsers and devices, it almost certainly points to a server-level issue. In such cases, collaboration with your hosting provider or system administrator is critical.

Although 501 Not Implemented errors are typically caused by server-side limitations, client-side factors - particularly browser behavior - can sometimes trigger or expose these errors. Before jumping into server troubleshooting, it’s worth verifying that the issue isn’t originating from the browser.


Common browser-related triggers include:

• Outdated browsers. Older versions may not fully support modern HTTP standards or correctly handle newer HTTP methods like PATCH or DELETE. This mismatch can result in unexpected 501 responses even if the server is fully capable.
• Extensions or add-ons. Certain browser extensions, especially those that modify headers or manipulate requests (e.g., ad blockers, privacy tools, or developer tools), can alter HTTP requests in ways the server does not recognize.
• Proxy misconfigurations. Built-in or system-level proxy settings can modify request routing, headers, or methods. If the proxy does not support specific HTTP verbs, the server may return a 501 even though the origin server itself is correctly configured.

Custom headers or non-standard requests. Some developer tools or automated scripts running in the browser may send unusual headers or methods, which the server may not implement, exposing a 501.

1. Test the site in multiple browsers and devices to determine if the error is reproducible across environments.
2. Temporarily disable extensions or run the browser in a “clean” mode to rule out add-on interference.
3. Check and, if necessary, reset proxy settings in the browser or system network configuration.
4. Update the browser to the latest stable version to ensure full support for modern HTTP standards.

By isolating browser-specific factors, you can avoid unnecessary server-side interventions and quickly identify whether the 501 error truly reflects a server limitation or a client-side anomaly.

While 501 errors deserve close attention, they are only one part of the wider 5xx family of server-side problems. These errors can affect a specific page or the entire website. Businesses should be aware of their cousins:

• 500 Internal Server Error, the catch-all for unexpected failures.
• 502 Bad Gateway, which arises when one server in a chain receives an invalid response from another.
• 503 Service Unavailable, usually caused by overload or maintenance.
• 504 Gateway Timeout, indicating that a server did not respond in time.

Monitoring the url of your website with tools that send HTTP requests can help detect and diagnose these server errors as soon as they occur.

Each tells a slightly different story about what went wrong. Understanding these differences helps businesses react more quickly and reduces downtime.

Although 501 Not Implemented errors are relatively uncommon, they can bring eCommerce operations to a halt when they occur. The root cause almost always lies in the server’s inability to process specific HTTP methods, making this a configuration- or infrastructure-level issue rather than a simple application bug.

For online businesses, the key to minimizing downtime and lost revenue is a hosting environment that fully understands and mitigates 5xx errors. Proactive monitoring, up-to-date server software, and proper configuration of HTTP methods are essential to prevent 501 errors from affecting your customers.

At Scalesta, we help eCommerce platforms stay resilient, responsive, and profitable - even under heavy traffic. By ensuring your infrastructure supports modern HTTP standards, handles uncommon request methods, and integrates seamlessly with CMS platforms and plugins, we protect your business from disruptions caused by server-side errors.

What does a 501 Not Implemented error mean?
A 501 error indicates that the server does not support the HTTP method used in the request (such as POST, PUT, PATCH, or DELETE). Unlike temporary errors like 503, it signals a missing or unsupported server feature rather than a transient failure.

How is a 501 different from a 500 Internal Server Error?
While a 500 error is a generic server-side failure, a 501 specifically points to unrecognized or unsupported HTTP methods. It usually reflects a configuration or software limitation rather than an unexpected runtime problem.

Can a browser cause a 501 error?
Yes. Outdated browsers, interfering extensions, misconfigured proxies, or non-standard request headers can sometimes trigger a 501 response, even when the server itself is correctly configured. Testing across multiple browsers and disabling extensions can help isolate client-side causes.

What are common server-side causes of a 501 error?
Typical triggers include unsupported HTTP methods in older server versions, misconfigured .htaccess or server config files, incompatible CMS plugins or modules, and intermediary layers like reverse proxies or load balancers not forwarding requests properly.

How can I fix a 501 Not Implemented error?
Resolution generally involves: updating server software, reviewing Apache or Nginx configurations, ensuring proxies or load balancers correctly forward requests, and verifying CMS plugins or modules are compatible with the HTTP methods being used.

How can eCommerce sites prevent 501 errors?
Regularly update server software, CMS platforms, and plugins; monitor HTTP requests; configure HTTP methods correctly; and work with a hosting provider experienced in 5xx error management. Proactive infrastructure management minimizes the risk of downtime due to 501 errors.