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Discussion Starter · #1 ·
I have permanent P0128 error code. Temperature Below Thermostat Regulating Temperature.
I found that thermostat should open around 95C. Coolant temperature reaches max 82C as shown on dashboard menu. I noticed also that after a few kilometers large pipe going to radiator is hot. It should be cold till thermostat opens.
Is this temperature reading and other observations means that thermostat is constantly open and should be replaced?
 

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The wife's '06 Chevy Trailblazer was giving this same code. Replaced the thermostat and that took care of it. Your thermostat is most likely stuck open and not allowing the engine to reach operating temperature within the time that the PCM thinks it should. And I'll bet your thermostat is easier to replace than the one on a Chevy 4.2L I6 engine!
 

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Discussion Starter · #4 ·
New thermostat did the work. Now the temperature reaches 90C and before that the main pipe is cold.

In the old one I cannot see if it stuck open. It look the same as new.
 

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Mine did the same thing. I don't know if the stuff in the thermo that expands maybe partially leaked out or its properties changed due to coolant getting in and contaminating it. Don't know, just guessing.
 

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Mine did the same thing. I don't know if the stuff in the thermo that expands maybe partially leaked out or its properties changed due to coolant getting in and contaminating it. Don't know, just guessing.
They don't work well when made by an anti-waxer. :)

Wax thermostatic element

Understanding and Testing Thermostats -EricTheCarGuy

Weak spring?
 
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I still don't see an explanation as to why it will close when cold, but open too early. Maybe I just didn't read thoroughly enough.
 

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Probably a weak spring. System pressure may push it open, then back to normal when cold and no pressure.
You said yourself that coolant pressure pushes the thermo closed.


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You said yourself that coolant pressure pushes the thermo closed.


View attachment 63580
Yes, I was correct the first time.

Spring is on the engine side.
 
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