This article explains how to find if you have pages with canonical tags pointing to pages that return an HTTP 404 error.
Overview
A canonical tag pointing to an error page is one of many reasons why search engines ignore canonical tags. Regularly evaluate whether your canonical tags point to error pages to ensure good site health.
Creating a Report of Canonicals Pointing to Errors
To check if there are canonical pages that return HTTP 404 errors, navigate to the "HTTP Status Codes Distribution" chart in SiteCrawler's HTTP Codes report:
Click on the 404 section of the chart to view a URL Explorer report of pages that returned 404 HTTP status codes in Botify's crawl.
Add a filter with the following field to include only pages with another page pointing to them through their canonical tag: "Canonical from Exists".
Add a report column to show which pages the incoming canonical tag comes from:
The resulting report shows the pages that have a canonical tag (in the "Canonical From" column) pointing to the page in the first column ("URL Card") that returns an HTTP 404 status code.
You can export this report to have the full list of pages with canonical tags pointing to error pages to be resolved.