2001 Chevy S-10 P0128 OBD ENGINE TROUBLE CODE

I AM TRYING TO SMOG MY TRUCK, BUT THE SERVICE ENGINE LIGHT IS COMING ON, GIVING A P0128 OBD ERROR CODE. THE RUNS VERY COOL, DOESNT SEEM TO BE HEATING UP LIKE IT SHOULD. THE CODE READER SAYS THE COOLANT TEMPERATURE IS BELOW THE THERMOSTAT REGULATING TEMPERATURE. ANY IDEAS ON WHAT WOULD MOST LIKELY BE THE CAUSE OF THIS? IM SURE ITS MORE THAN LIKELY A COMMON PROBLEM FOR THIS MAKE MODEL AND YEAR.