Hmm, I had read that Volvo invented this radiator system a while back but didn't know that it was being used.

This makes for an interesting issue. Can you be held liable ( through an emission test ) for an emission component that does not reduce the emissions from your own car?

Get a hold of a wiring diagram, and see if this is a 2 or 3 wire sensor. You might be able to use something else. I also don't see how a temperature sensor can tell the difference between a regular and cat rad.