If you see this error in your Site Audit report and not sure what that means, you are in the right place:

What is canonical link?

Just in case you don't know; canonical links are used to solve duplicate content issues. If you have several pages with the same or similar content, you need to pick the one that you want to rank.

And by pointing canonical links to that page from its copies, you explicitly tell search engines that this is the page they should index (and hopefully rank) instead of the others.

A common use case of canonical link is, for example, product variants in eCommerce shops. 

Here's a quick example of how it can look like:

<link rel="canonical" ">href="http://example.com/">

If you want to find out more about them, feel free to check Google's guide.

What happens if your canonical links point to pages with 4XX code?

This issue indicates that there are URLs specified as canonicals on the pages of your website that return one of the 4xx HTTP status codes. Which basically means that the page isn't accessible. And if it's not accessible for search engines, they will be unable to index it and it won't show up on the search results page.

There are various types of 4XX codes, you can check their description here:

List of HTTP status codes

What should you do?

Some of the issues can be easily solved. Some are trickier, and qualified assistance is highly advised here.  

But here's a brief overview of the 4XX HTTP issues you are likely to deal with. 

400 - Bad request

This error stands for communication issues between the server and your browser. Basically, the server failed to understand the request your browser is sending. 

This type of HTTP code can be caused by errors in the URL, the syntax of it. You might want to check the URL in the rel=canonical for non-allowed symbols, like a percentage character, etc. 

Here’s a list of unsafe URL characters.

401 - Unauthorized 

It’s a permission issue that indicates the page is accessible only for logged in users. As you know, canonical links are meant to rank. Seeing that that page is publicly unavailable and you still want it to be so, you should either remove the canonical link to it or find the page that better suits this purpose.

403 - Forbidden

It has to do with permissions as well and means that the content is blocked for a specific user group. 

You can grant free access to it via your server or remove/replace the link. 

404 - Not found error

Probably the most common 4XX HTTP status code out there. The page was either deleted or its URL has changed. Possible ways to fix it:

  • make sure the URL in rel=canonical is the correct URL of the canonical page. It might have a typo.
  • if the canonical page is gone, find or create a new one and set it as canonical; refrain from redirecting the old page to the new one as it will result in 'canonical points to 3xx' issue. 
  • consider removing the link at all if you don’t have any substitution for the missing page. 

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