2005 Sierra, I had to change the engine wiring harness and now the temperature display in the mirror shows "OC". I am guessing OC is for open circuit but the compass works fine and is accurate. All the connections between the engine harness, body harness, and fuse block are good.

I found something on the Bing/google machine that says the mirror will display "OC" if the mirror senses a temperature of less than -40°C (-40°F).
the mirror will display a "SC" if the mirror senses a temperature of greater than 112°C (143°F). It is a bit chilly out but it damn sure is not 40 below.

Any Ideas?