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Username Post: '08 Trailblazer PO128 code help?        (Topic#366332)
8683jb 
Poster
Posts: 91

Reg: 09-06-13
09-05-21 05:40 PM - Post#2826601    

My '08 Trailblazer with the 4.2L I-6 engine was throwing a P0128 code with the typical temp gauge going to zero and A/C shutting off. I changed the pain-in-the-butt-to-get-t o temperature sensor and it was fixed for a few weeks, but it's back again. I checked the resistance of the temp sensor and when it's cold it reads around 1800 ohms and when hot it reads about 300 ohms, so I think it's working as it should. I'm pretty sure it's not the thermostat because the engine is definitely at running temp when it throws the code and the gauge goes to cold, and when I clear the code the temp gauge immediately reads properly. What else should I do to fix this?



 




gchemist 
Member #271 Senior Chevytalk Moderator
Posts: 25529
gchemist
Loc: Austin, TX 78748
Reg: 05-09-00
09-06-21 03:07 AM - Post#2826614    
    In response to 8683jb

Is the clutch fan engaged? For its age, it may be engaged all the time causing excess cooling. The thermostat could be stuck open. The code is more emissions related than engine operating. How old is the thermostat?

'83 Silverardo XST - ZZ4 powered
'95 Jimmy SLT (Bought @131,814)
'96 GMC Jimmy LS Ret. @236651 miles
'08 GMC Acadia 3.6L SLE


 
Shepherd 
Frequent Contributor
Posts: 2302

Loc: Lake George, NY
Reg: 11-11-15
09-06-21 06:15 AM - Post#2826631    
    In response to gchemist

I had a TB, went thru 2 tstats over it's life, this was a common problem as was the electronic fan clutch.



 
8683jb 
Poster
Posts: 91

Reg: 09-06-13
09-06-21 06:49 AM - Post#2826634    
    In response to gchemist

The thermostat is original. I'd change it in a second, but I really don't think it's the problem. When the light comes on and the gauge goes to 100, its lowest indication, the water is showing to actually be at 176 degrees using the scan tool. Wouldn't the water temp be colder if the thermostat was the fault? Is 176 degrees actual temperature low enough to trigger the code? The fan clutch seems to be working normally.



 
Shepherd 
Frequent Contributor
Posts: 2302

Loc: Lake George, NY
Reg: 11-11-15
09-06-21 12:46 PM - Post#2826650    
    In response to 8683jb

It is a 195 stat, if fully warmed up it should run around 200. The Algorithm in the pcm is probably programmed to see 190 minimum fully warmed up. As I said this was common in those trucks.



 
8683jb 
Poster
Posts: 91

Reg: 09-06-13
09-06-21 01:21 PM - Post#2826651    
    In response to Shepherd

OK thanks Shepherd. I'll pick one up and see if that cures it. I appreciate the help very much.



 
Shepherd 
Frequent Contributor
Posts: 2302

Loc: Lake George, NY
Reg: 11-11-15
09-06-21 02:52 PM - Post#2826655    
    In response to 8683jb

Don't forget to bleed the system when refilling, use the coolant bleeding port.



 
8683jb 
Poster
Posts: 91

Reg: 09-06-13
09-06-21 04:02 PM - Post#2826658    
    In response to Shepherd

Will do. Thanks again.



 
gchemist 
Member #271 Senior Chevytalk Moderator
Posts: 25529
gchemist
Loc: Austin, TX 78748
Reg: 05-09-00
09-07-21 03:09 AM - Post#2826668    
    In response to 8683jb

GM had fun using two temp probes. One for the gauge. Another for engine use. The block sensor between cylinders can be hard to reach.

'83 Silverardo XST - ZZ4 powered
'95 Jimmy SLT (Bought @131,814)
'96 GMC Jimmy LS Ret. @236651 miles
'08 GMC Acadia 3.6L SLE


 
8683jb 
Poster
Posts: 91

Reg: 09-06-13
09-07-21 11:16 AM - Post#2826684    
    In response to gchemist

I replaced the thermostat this morning (no picnic either) and so far, so good. No fault light yet, and hopefully it'll stay that way. The temp gauge reads 210 now where it was showing about 180 before, which I thought was normal. Now that I see 210, I remember that's where it always ran, so it was indicating a lower-than-normal temp. Too bad I wasted the effort, expense and vocabulary replacing a sensor that was probably fine. Thank you guys again for your help with this. I sure appreciate it.



 
gchemist 
Member #271 Senior Chevytalk Moderator
Posts: 25529
gchemist
Loc: Austin, TX 78748
Reg: 05-09-00
09-08-21 05:36 AM - Post#2826734    
    In response to 8683jb

Good fix. Yea, 210 is normal for aluminum engines. Good luck.

'83 Silverardo XST - ZZ4 powered
'95 Jimmy SLT (Bought @131,814)
'96 GMC Jimmy LS Ret. @236651 miles
'08 GMC Acadia 3.6L SLE


 




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