tempC = ((5.506 - sqrt(sq(-5.506) + (4 * 0.00176 * (870.6 - tempmV))))/(2 * -0.00176)) + 30; //LMT84 temp sensor transfer function
You must be close, because when I look at the data sheet http://www.ti.com/lit/ds/symlink/lmt84.pdf the lookup table shows that you should be between 28 and 29 C for the 879mv shown in the printout.