Chevrolet: Chevy Blazer, chevy blazer, temperature gauge

Question
I have a 1999 Chevy blazer.  I am having trouble with the outside air temperature gauge.  After having some work done the gauge was messed up. It could be 30 degrees outside and the gauge will say over 100.  The only thing that works is the compass.  I bought a chevy manual, and it says I need a code to reset the gauge.  However I haven't been able to find them.  Any help would be appreciated.

Answer
James;
Hi well if it was worked on the gauge that-could have been damaged, if so that will or can cause you to get false readings.James- I looked thru all of the ( OBDII ) booklets to try to find your data, but w/o a read out I cant find any
thing- can you get a meter to get me some codes to pop up  
than I can get you some readings, as if I have no read outs I have no ideas what is wrong.w/o the obdII#s