How to find source of "Page Not Found"

[InvincibleRecordings]

Since our new site was ungraded to the Shadows template from a rather old Miva template from about 10 years ago we are getting a lot of "Page Not Found" experiences from users. We had none before the upgrade. I have been searching Google analytics and Web Master to try to find a way to drill down and see what people are going to on our site to get this "Page Not Found" but have been able to figure out how to track the actual causes. I am hoping someone knows the way to get this figured out.

[GDesigns]

When did you launch and do you still have a backup of your old site. I assume your site structure either changed or you eliminated a bunch of old pages/products?
Also - check Google Search Console for the 404 pages/origins, you should be able to export that list. That should give you a good idea.

[InvincibleRecordings]

I must be doing something wrong. In google console it is showing zero errors when I do a crawl. But when I go to google analytics is is show the number one landing page as 404 error pages. Regarding changing a lot of categories or products no they are the same on the upgraded site.

[Bruce - PhosphorMedia]

Though you should have these errors logged in GA, you can also use this tool:

https://www.screamingfrog.co.uk/seo-spider/

To go through your site. Its a handy tool to have for a variety of SEO related testing and data collection and a lot easier to get useful data dumps than GA.

[AHerb]

In addition you can pull all 404 errors that were already found by your customers from the error.log file on your server.