Help needed PO128 code

[zieg85]

Need direction and thoughts regarding a check engine light.  PO128 is the code.  This is my wife's 2006 Buick Lucerne.  The car runs fine, heats up the way it should, at least the temperature gauge runs midway like it always had done.  The heater will run you out and it is warm before you know it.  I have already changed the Coolant Temp sensor.  I am at a loss of what else to look for.  Any help would be greatly appreciated.

[bake74]

     Have you checked your thermostat, 3 different places I just went to and read said this was the number 1 cause for this code.
     Also mentioned the thermostat might be leaking by, and faulty cooling fan, running all the time instead of coming on when needed.

[zieg85]

I have read the same things as well.  Haven't checked the thermostat, just going by the way the car comes up to temperature fairly fast.  I guess it is a cheap enough thing to try.