Tracking-down a circular reference in Creo 4

If you look in the directory where you are working, you'll notice that whenever Creo finds such circular references it writes a file with the extension ".crc". That file will list all the offending features that comprise the circular reference. Hopefully the information provided in the file will be helpful - if I remember correctly it references Feature ID numbers.

As mentioned by @StephenW , the reference viewer is the tool for this. These snapshots are from part mode, but it works the same way in assembly mode.