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Q: Check Engine light P0128 Running to cool on 2004 Chevrolet Trailblazer

From all my research, it seems that this code mean I -(hubby) needs to replace the thermostat. Seem like it may be stuck open. Trailblazer doesn't get driven to often, it has approx 50,000 miles on it. Haven't really notice much difference in how it drives when the light is on.. One or twice it ran a little rough, but nothing extreme. The three times the light has come on, it has been raining and cool out, or late at night when it is cooler. Also all three times we were going around 50 mph.
Is this easy to do that a DIY-er could do, or is this an issue for the dealership? We paid it off, and 3 days later the light comes on.. Figures.. Like everyone one else we have no extra money right now.. Input would be great.. Where to look for the thermostat housing, and any trick or tips..
thanks for any help!
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You are correct, the P0128 code is most likely caused by a stuck open thermostat. Believe it or not a low coolant level can also cause the P0128. So, check the coolant level with the engine cold, make sure the radiator is full, if not top it off and also the coolant bottle if necessary. You will then need to monitor the system for leaks and resenting of the p0128. If the radiator is full replacing the thermostat would be the next step. The thermostat on your 2004 Chevrolet Trailblazer is located in the side of the engine block behind the alternator. My service information suggests removing the alternator to gain access to the thermostat housing. Your hubby will need to determine for himself if he thinks he is up the task of replacing the thermostat. If the decision is to take this job on please take extra care to make sure to get all of the air out of the cooling system when refilling it.
He is mechanically inclined, although I guess I didn't say that.. sorry Jeff! LOL He was a manager of a oil lube shop for many years. he is not completely going into this blind. We have checked the levels and they are perfectly on the lines where they should be. So that is why he thought it had to be stuck open..

Thanks for the advice... I have one more questions.. is it going to hurt to drive the car short distances until we can get it fixed next week. We may put 30- 50 miles at most on it in a week.

Thanks again
To run the car when the engine is running too cool is not going to be an issue other than fuel economy. The object of the thermostat is to heat the engine up to burn the fuel at the most efficient rate possible. The hotter the engine the more completely all the fuel burns thus increasing the efficiency of the engine and maximizing fuel economy.
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