If you’ve seen a “501 Not Implemented” message, your server is telling the browser: “I received your request, but I don’t support the function needed to handle it.” It’s less common than a 404 or 500, but it still disrupts sales, enquiries and trust. Here’s the plain-English version and a practical plan to resolve it quickly.

What “501 Not Implemented” really means

A 501 is a server-side error. It usually appears when a request uses an HTTP method (like PUT, PATCH or DELETE) that the server or an intermediary (CDN, proxy or web application firewall) doesn’t support. That’s different from:

• 405 Method Not Allowed: the server knows the method but has disabled it for that resource.
• 500 Internal Server Error: a generic failure while processing.
• 503 Service Unavailable: a temporary overload or maintenance message.

One quirk: 501 responses can be cached by browsers or proxies. If a cached 501 lingers, it may look like nothing has changed even after you’ve fixed the underlying issue.

How it appears to visitors

Wording varies by platform, but you’ll typically see:

• “501 Not Implemented” or “HTTP 501 – Not Implemented”
• Sometimes a branded error page, sometimes plain text — the key is the 501 code.

Common causes

• Unsupported methods: An app, plugin or integration sends PUT, PATCH, DELETE or WebDAV verbs that aren’t implemented on the server.
• Missing modules or features: Required server capabilities (e.g., URL rewriting) are disabled or absent.
• Edge layer limits: A CDN, reverse proxy or WAF blocks or mishandles specific methods or headers.
• Routing or DNS mistakes: Traffic reaches the wrong host or a default vhost that doesn’t know your site.
• Rewrite rule issues: Broken .htaccess or equivalent rules sending requests down unsupported paths.
• Outdated software or conflicts: Older server builds or app/plugins assuming capabilities that aren’t there.

Business impact

• Lost trust and conversions: Visitors can’t read pages, submit forms or check out.
• SEO concerns: Extended 5xx errors can slow crawling and affect rankings. Unlike a 503, a 501 implies missing capability — not a brief outage.

Quick checks for visitors (harmless and fast)

• Hard refresh: Force a fresh load (Ctrl/Cmd + Shift + R) to clear a cached error.
• Try another browser or private window: Rules out extensions and stale cache.
• Test another device or network: Helps identify local proxy/filter issues.
• Use an external checker: Confirms if the problem is widespread (server-side).

Practical steps for site owners

1. Map what’s failing: Is it site-wide or just during actions like form submits or API calls? Note when it started and any recent changes (plugins, theme, CDN/WAF rules, DNS or deployments).
2. Inspect the request: In your browser’s Network tab, reproduce the error and note the method (GET/POST/PUT/PATCH/DELETE) and endpoint. If a plugin is using an unsupported method, disable it and retest.
3. Undo recent changes: Roll back new plugins/themes or code. Restore known-good rewrite rules and test again.
4. Bypass the edge temporarily: If you use a CDN or WAF, route a test request directly to origin. If the 501 disappears, review method restrictions, header normalisation and custom caching at the edge.
5. Verify DNS and routing: Ensure records point to the correct origin and proxies forward to the right upstream.
6. Escalate to hosting: Ask your provider to confirm enabled modules (e.g., rewriting), check server logs for “not implemented” responses, and validate method support your site relies on.
7. Purge caches and confirm: Because 501s can be cached, clear browser, proxy and CDN caches once fixed, then monitor for any recurrence.

Prevention that works

• Stay current: Keep servers, CMS, plugins and themes updated as part of routine website maintenance.
• Favour standard methods: Use widely supported HTTP methods and confirm end-to-end support for anything unusual.
• Test in staging: Trial plugin/theme updates and CDN/WAF rule changes before going live.
• Security monitoring: Track 5xx spikes and failed endpoints so you can act quickly.
• Reliable edge services: Use reputable CDNs/WAFs with sensible method policies and clear rollback paths.
• Backups & rollback: Keep automated, off-site backups for rapid recovery alongside malware protection and malware removal readiness.

How matm can help

• Managed WordPress, plugin & theme updates
• Security monitoring and WAF setup
• Regular backups & fast site recovery
• Malware removal and emergency response

Need calm, practical support to fix errors and strengthen your WordPress security? Email [email protected] or call 01952 883 526.

Based on research by Sucuri — read the original analysis.