Page Not Found errors or '404s' are a bad experience. The Google Search Console bot might have sent you an email warning you that there's been an "Increase in “404” pages on example.com" with reference to Googlebot for smartphones.
The email looks like this:
If you've got a Wordpress website or similar, the cause of this might be that smartphone (mobile) visitors are being directed to a different version of your website than other. That's the first thing to check, also log into your webmaster tools account to see the list of 404s causing the error.
How to fix it: Log into Search Console
Log into your Google Search Console account (sometimes called webmaster tools) and navigate to the crawl errors section.
Here you'll see a list of pages on your website which Google used to be able to go to, but no longer work. It you're job to either set-up a redirect to a new page (there are many plugins for wordpress that help with this) or, the same can be achieved by editing your .htaccess file. Use a plugin if you can, and unsure what you're doing. If the page has been deleted and you don't want it directing anywhere new then leave it be. Or create a custom 'not found' page which apologises to your visitors but instead turns it around into an opportunity to show them relevant content they might otherwise be interested in.
You can stop here, the rest is additional information.
How else can I find these not found pages?
The email is complaining solely about mobile users. A poor mans solution would be to whip out a mobile phone and navigate your website manually to test it- because depending upon how your website is set-up your laptop/desktop might be showing you different pages when visited from a larger screen display. It's best to use the Search Console Tool described above.
Another fun way is to use the Google "site:example.com" feature. This will tell Google to show you all the pages it currently lists in it's index about your website.Go through them and make sure they all go somewhere and not 404s! - Using this way would only make sense if you've not got access to your webmaster tools for some reason, and you really should.
It looks like this: How to tell Google to show you all the pages it currently has in its index for your own website only:
Why would I bother doing this?
Your website builds up traffic overtime , people link to it and if those links die (become 'not found' then it's a bad experience and eventually all those pages will be removed from search engine indexes. Why? Because search engines would be pretty useless if they just directed people to 'page not found' errors. You've spent a lot of time generating this content, so if you've got time, fix the error and/or point the visitors to the updated, or a new relevant location if there is one.
This post was inspired by beiwei55.com, which offers UK Chinese tours by Mandarin speaking British guides whom we occasionally support.