2001 Chevy S-10 PO128 nightmare please help

I have a 2001 Chevy S10 Xtreme Pickup

The light came on and the code was PO128.

I took it to a shop and they replaced the thermostat- the light came back on.

I took it back they replaced the sensor - the light came back on

I took it back they replaced the thermostat again- the light came back on

coolant level is fine- fan isnt running excessively etc.

took it to another shop- they replaced the thermostat and sensor again bc everyone says nothing is wrong with the truck- yep you guessed it the light came on again. WTF!

The 2nd shop also replaced the radiator cap.

someone suggested maybe it was the onboard computer? The truck is NOT over heating but I cant have the light on and be able to pass emissions to get my tag.

Im seriously at my wits end here anyone have a clue what it might be? Its always the same code (had several different places check the code)