Close gaps

[Nick W]

I have imported a complex and large building footprint and would like to make a simple extrusion to represent the building. However, when I try to make a face using the "create face" ruby, it says that the "selected edges do not form a closed loop". I presume that there is a tiny gap somewhere and would like to avoid scouring the whole perimeter to find that gap.

I thought that I had come across a discussion on this forum about a ruby that would "clean up" an imported CAD drawing. I have one called D.B. Tools (Didier's, I presume) that has a "clean selection" function, but this just deletes my whole selection.

Am I not understanding how to use this tool, or am I using the wrong tool?