2004 Chevy Trailblazer P0128 code:

The only thing that code is telling you is that there could be a problem in that circuit somewhere or the circuit is accurate and the engine really isn't getting hot enough and it would take detailed troubleshooting to be sure about anything. It can be the sensor, the PCM and all related wiring to the sensor. In this case it can also be a bad thermostat. What you need to do is determine the actual temp of the engine using a laser thermometer or something like that. You should also use a scan tool to determine what temp the PCM is seeing before making any decisions on what to change.

A bad thermostat will cause this code. Have that replaced, and have the code cleared before you go changing anything else. If the code persists, then it may get expensive.

I do believe I just said that.

What I suggested is to determine the actual temp before changing anything. I wouldn't go throwing any parts at it until the real temp is determined.

Well a 9 dollar thermostat which will solve the problem 95% of the time isnt a bad place to start. Before "throwing" more expensive parts at it.