HTTP 501 Not Implemented Error: What It Means & How to Fix It

We go over what a 501 Not Implemented error means, why it shows up, and the steps you can take to troubleshoot and prevent it.

501 Not Implemented error code meaning

A 501 Not Implemented is a server-side HTTP status code indicating that the web server does not support the functionality required to fulfill the request. In most cases, the server recognizes the request method but cannot carry it out due to unsupported or unimplemented functionality in its configuration or software. For users, this appears as an HTTP error response, while for developers, it signals that the server needs to be updated or reconfigured to support the required HTTP methods.

Common causes

There are a few technical issues that commonly trigger a 501 error, one of the most common being an unsupported HTTP method or request header. This might happen when a client makes an API call to an endpoint that hasn’t been implemented on the server. If the method is recognized but not permitted, the server may instead return a 405 Method Not Allowed error. Incorrect Content-Type values or missing HTTP headers can also create communication issues between the client and server.

Misconfigurations in the server setup and outdated server software can also lead to 501 errors. If the server isn’t properly configured to handle the request or lacks support for newer HTTP methods or protocols, it won’t be able to process the client’s request.

Plugin conflicts and compatibility issues within the server environment can lead to 501 errors as well. Conflicts caused by things like outdated plugins or overlapping functionality can result in a variety of error types, with 501 errors being one of the most common.

Troubleshooting 501 errors

Troubleshooting a 501 error depends on who’s addressing the issue. Users can start with simple fixes like refreshing the page, clearing cache and cookies, switching browsers, or disabling proxy settings. Developers should focus on checking supported HTTP methods, updating server software, reviewing configuration settings, and analyzing server logs to identify the root cause.

Below, we’ll provide clear instructions for both users and developers to help fix the problem:

For users

Try reloading the page

The quickest and simplest solution is always the one you should try first, and reloading the page that returned the 501 error may fix the problem. If the error is caused by a temporary glitch or network issue, simply hitting the refresh button on your web browser will often solve the problem.

Clear browsing data

Clearing your browsing data, including cookies and cache, will often resolve 501 errors. The reason is that cached data can sometimes become corrupted or outdated, which leads to errors when loading web pages. By clearing your browser cache and cookies, you can eliminate any corrupted or outdated data and ensure you’re using the most up-to-date version.

Try a different web browser

Switching to a different browser can help determine whether the issue is browser-specific. Sometimes, outdated settings, extensions, or cached data in your current browser can interfere with requests. Testing the site in another browser, such as Firefox, can help determine whether the issue is browser-specific. Also, check if the site relies on HTML forms using nonstandard methods that your current browser may not support.

Disable proxy settings

If your browser is set up to use a proxy server, it may interfere with your ability to access certain websites and can trigger 501 errors. To see if this is the issue, try disabling proxy settings in your browser and attempt to load the page again.

Contact website owner or hosting provider

If none of the methods listed above solve the issue, consider contacting the website owner or hosting provider for further assistance. They will have access to server logs and advanced diagnostics tools that can help identify the error’s root cause. To expedite the process, be sure to provide all the information you can about the error, as well as the troubleshooting steps that you’ve already taken.

For developers

Check the request method

A 501 error often occurs when the server doesn’t support the HTTP method used in the request. Review the method being sent (such as GET, POST, PUT, or DELETE) and ensure it’s supported by the web server. Try using tools like Postman to confirm which HTTP method and headers are being sent. On platforms such as Apache, Nginx, or Microsoft IIS, verify that server modules or handler mappings allow the methods your application or API requires. Reviewing documentation or the relevant HTTP RFCs can also help confirm correct behavior.

Check for software updates

501 errors are sometimes caused by outdated software that needs to be updated. Keeping your software up-to-date is a good practice even if you aren’t encountering 501 errors, and it’s certainly something you should do if you are.

Monitor server logs and messages

You can get a lot of valuable information from server logs and messages regarding the root cause of a 501 error. Start by checking for messages related to the request that triggered the error, then look for details about which functionality or method the server doesn’t support.

Review and test with minimal configuration

Review your current setup and test the site with minimal configuration to help isolate the cause of a 501 error. Whether on your WordPress site or another CMS platform, disable unnecessary plugins, revert to default settings, and simplify your setup as much as possible. If the error resolves, re-enable components one by one, reviewing and testing after each change to identify what’s causing the issue.

Optimize server-side performance

Implementing server-side optimizations can sometimes eliminate 501 errors while also improving your website’s overall performance. It’s also important to review your website’s code to ensure it’s clean, well-structured, and free of syntax issues, as this can greatly reduce the likelihood of future errors.

How to prevent 501 errors

Preventing 501 errors from ever occurring in the first place is a whole lot better than trying to fix them after the fact. Thankfully, there are several things you can do to improve your website’s performance and reduce the likelihood of 501 errors.

Regular maintenance and updates are an essential place to start. Things like vulnerability patching, ongoing monitoring, and regular updates are all major keys to preventing 501 errors and keeping your website healthy in general. It’s also a good idea to implement robust testing procedures so you can identify and address potential compatibility issues proactively.

Optimizing your website’s code via methods such as using a content delivery network (CDN) can also help. You should also strive to reduce your dependency on deprecated features and always adhere to website coding best practices.

Lastly, enhancing server security is key to preventing 501 errors (among a number of even bigger issues), and many of the steps taken to boost a website’s security will improve its performance as well. Using a web application firewall (WAF), for example, will block malicious traffic and prevent it from harming your website’s functionality.

Sustain a healthy website with SiteLock

501 Not Implemented errors can disrupt normal site operations and leave users frustrated with their experience. Identifying the root cause and fixing it quickly through targeted troubleshooting is the best way to resolve these issues. Keeping your site secure and well-maintained can also help minimize factors that contribute to unexpected errors or unstable behavior.

SiteLock offers website security and performance services that help maintain a clean, stable environment across platforms like WordPress and Drupal. With ongoing scanning, malware detection, and WAF, SiteLock supports the long-term health of your website.

Learn more on how SiteLock works and review our pricing options to find a plan that fits your business needs.

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