2005 Chevy Trailblazer low coolant temp

My check engine light keeps coming on (and thenshuts itself off) finally it stayed on long enough to get the code read and the code is 128 (low coolant temp). The temp gauage works just like always and in a timely manner goes up to 210 and stays there. No excess fuel use, no hesitation, no noises.
What might this be - what might need done and what is a ballpark cost.