2002 Chevy Trailblazer - Rear View Mirror Temperature Reading 0C

I belive OC means open circuit suggesting there's a break in the circuit to the sensor. Might try unplugging or moving around the sensor. It's behind the grille. I think left of center.

Would the grille need to be removed or is it reachable?

I really don't know. I've got an Envoy. First suggest locating the sensor. It should be the only thing mounted fairly high behind the grille with wires going to it. The Envoy grille is fairly easy to remove but not sure about the TB. Best guess is it would be easier to get to with the grille out of the way.

Thanks For Your Help!