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Sounds like either a bad thermostat or a faulty coolant temp sensor. I'm not sure how hard either of these are to replace. The thermostat is located on the serpentine belt side of the engine where the radiator hose goes on the 3.5L. You may need to go to the dealer to get the thermostat. I'm not sure where the coolant temp sensor is located on the engine. On most GM cars they are located somewhere on the intake mainfold. Any autoparts decent autoparts store should have this sensor in stock for under $30.
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