Q: 2004 TB P0128 OBD Code on 2004 Chevrolet Trailblazer

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I have had this P0128 Code for over a year now - so it obviously is not causing problems because the engine runs fine, great gas mileage, etc. The problem says that the thermostat should be replaced; but that is very involved and expensive. Are there any other things I can try? It seems strange the the Check Engine light would stay on (even after reset) for the same thing yet not cause any obvious problems. I know TBs have instrument cluster electric problems.
(1) Answer
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it does not say the thermostat needs to be changed. what the code identifies is that the engine is not running at the temperature it is supposed to run at. the coolant sensor may be at fault or wiring or maybe the t stat. that is what a diag will tell you.
get it checked by a shop instead of throwing parts at it.

Hello, Roy - I made a mistake by saying that is what the code meant. I should have said that when I researched the possible reasons for the code, one of them was to replace the thermostat. I will get it checked by a shop as you suggested.