When you open your WordPress sites RSS feed in the browser you get the following error:
“WordPress Feed Invalid Char”
The error will also show you the the line number of the error.
View the source of the feed and go to the line of the error. Find the corresponding post in your WordPress admin.
Copy the title and body into notepad (note this will remove all formatting).
You will be able to see the non UTF8 characters in notepad and be able to delete them. You can then copy and paste the problem text back into WordPress.
The offending characters won’t show in the WordPress admin hence having to copy into notepad.