i replaced the thermostat and i am still setting p0128 code.what next.

What is your question?

The scan tool i have will not read sensors or tell me anything other than the code that has been set. i don't want to go to a mechanic and would like to know the most likely problem causing this code. i am not the one driving the car and am not sure when and under what conditions the code is being set, i think it is happening after the car is warmed up and been driven a while.