404 Checker: A Tool for Website Owners
We distinguish pages with hard and soft 404 status. Soft 404 pages return a 200 HTTP status code and show a message about a nonexistent page. Hard 404 pages have the same content, however, they also have a 404 HTTP status.
The Prerender service cannot interpret the content of the pages, so it relies on the HTTP status of the page. Only pages with 200 HTTP status are cached, other pages are only rendered if a search bot requests the URL.
So the HTTP status codes are important from the aspect of the Prerender service, especially 404 because Prerender won't cache hard 404 page.
The 404 checker
As a website owner or administrator, it's important to ensure that your website's 404 error pages work properly. That's where the 404 Checker comes in. It's a specialized web service that thoroughly analyzes your website to confirm that when a non-existent or broken URL is requested, your server responds with the appropriate HTTP 404 status code. This confirms that the requested resource was not found. You can add multiple sites to the tool.
When a domain is first added for verification, the tool will show a question mark icon under the DOMAIN REACHABLE and 404 STATUS RETURNED columns. This will last for the entire time the domain is being checked.
Then it will return a check mark under both if your site is reachable and it has a hard 404 status for non-existing URLs.
In order to verify that you respond with the correct 404 for non-existent pages, the tool will create a fake URL based on the domain added for verification. If the status is not 404 for the bogus URL that the tool requests, then the 404 checker tool will show a ⚠️ warning icon under the 404 status returned column and the tooltip of the icon will say "Failed, 404 not returned". This might also happen if the site is not available for the 404 checker. There is another indicator under the DOMAIN REACHABLE column that lets you know if the site is available for the checker.
You can select how often would you like to check your site with the 404 checker.
How to fix soft 404
With the prerender-status-code meta tag, you can let the Prerender service know what HTTP status code it should return for the bots.
So by adding the following meta tag to your page's <head> section, you can let the Prerender crawlers know that the page should be marked with a 404 status.
<meta name="prerender-status-code" content="404">
The HTTP status of these pages won't be changed by this when a human visitor requests the pages, however, it will change what HTTP status the search bots get.