2003 Chevrolet Trailblazer P0128.
4.2 Inline 6 cylinder
Service engine soon codes  P0128.

Question = My service engine light is on. The dealer said the code is P0128 "Coolant Thermostat" (Coolant temperature below thermostat regulating sensor).
My temperature gauge reads okay and the vehicle runs good. Is it difficult to change this myself?
(I do have some mechanical ability) If so, can you provide me instruction?
Answer: Thank you for the tip.
Yes, this is a common cause for P0128.
They are not too difficult, but a little tricky.
The thermostat is the actual thermostat and the housing as one unit.

Best way is like this...
Remove drivers side wheel. It is easiest to get through the wheel well.

Use a 10mm swivel socket or a regular socket on a universal joint. Remove the 2 bolts on the housing. Leave the hose connected to the housing.

Grab the bolts before they fall.

Then go on top and remove a 10mm bolt that holds the large A/C line on the top. This gives a little more room for next step.

Pull the housing / hose up so can work on it from the top. Much easier than trying to do everything from the wheel well.

With a marker, mark the hose and housing to index the housing. The hose can be very stiff, and doing this will ensure that when you put the hose and housing back down to the side of the engine, it will line up just perfect.

Now remove the hose clamp and hose from housing. Install the new housing in the same position using the mark on the hose. Now push hose back down, and it will line up just perfect to install the bolts through the wheel well. Be sure to scrape the matting surface on the block nice and clean.

I will send a pic of its location in a few minutes.
Chevy Trailblazer Thermostat Replacement- P0128

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