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Hey everyone, Ive been having a problem with my 03 Envoy that I haven't been able to figure out as of yet. So far my code on my car reads as P0128, and it has read this for about a year. I have replaced the thermostat on my car which i thought would fix the problem but it hasn't gone away. So i figured it might be a temp sensor reading incorrectly causing the light to come on, i replaced this also but to no avail. The light still comes on and reads P0128. My temp runs below 210 which is the normal so im not sure what it is. The car heats up great in the winter and Ive owned an 02 Trailblazer and it heats up about the same so im inclined not to think i have another bad thermostat so i don't know what could be the problem. Any ideas?
 

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2004 gmc
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What exactly does your gauge read when you're running normally and warmed up? How much below 210?

This is a simple code. You've swapped the normal items that should have fixed it. About all there is that you haven't changed is the PCM (expensive) and checking the wiring from the sensor to the PCM (visual inspection and unplugging the connectors should be enough.

To do anything more, you need a Scangauge or something that can read the ECT parameter as the PCM is measuring it. P0128 simply sets when the PCM sees an ECT under 176 degrees F when the engine's been running for a while and it KNOWS the coolant temp should be higher than that. If you're running at 160 degrees, for instance, the heater might work fine in the winter, but the engine is still not operating in its designed temp range.

The thermostat or the sensor might still be bad even after replacement, but we need to know the reading from the PCM to decide which.
 

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2003 chevy trailblazer_ls
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If you mean the larger line, that's horrible! No wonder the code sets. It should be within a needle width of 210. My previous troubleshooting advice still applies.
:iagree: Yep!
That is where mine sat forever and it did set a code. New t-stat fixed this and is now right on 210. Advise you do the hot water test on the old and the new t-stat. Far too much work to install another bad one.
 

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2004 gmc
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I have the software to do it for free. Just drive to San Diego.

Autozone will read codes, hoping you'll buy the parts there. They aren't allowed to dig deeper and run a operational diagnostic because that's not their job. In CA and some other states, they quit even reading codes because of lawsuits.

Diagnostic centers do it, but there's no reason to think they'll do it for free.

If you're a mechanic working on your car a lot, you should own one. If not, you have to budget to pay somebody who has made the investment. At some point, it's cheaper to own one.

A scan gauge is $160 on Ebay w/ free shipping, and some used ones are going for $100-120. If you're a member of a local trailvoy chapter, there's probably somebody with the tools in that group. Sorry - I can't advise anything more hopeful than owning a modern vehicle isn't cheap given the level of computer modules in them.
 

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2002 chevy trailblazer_lt_ext
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Dumb question, but are you sure you installed the correct thermostat? Some aftermarket manufactures sell a 180*F and a 160*F version of our thermostat.

Correct me if I'm wrong Roadie, but ours is spec'd at 195*F, correct?
 

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AFAIK, 195. Never knew about the lower temp ones. Since that would mess up the tune, I have no idea why anybody would offer those.
I don't know, either. I came across this when I replaced my thermostat- when I went to Advance Auto Parts to buy a replacement they gave me the choice of which temp I wanted. I always remembered the gauge sitting around 200*F, so I bought the 195*F.
 

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Replacing the thermostat in my 2004 Envoy

I have just purchased my 2004 Envoy SLE. I pulled a P0128 code and have purchased the thermostat. I am having a tough time getting to the bolts for the thermostat. I reading your discussions it sounds like I am making this a more difficult project than it may be. Any insight would be helpful. My temp guage is currently holding steady at about one and a half bars below the 210.

Thanks in advance, Roger
 

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Welcome. It isn't trivial, but unless you have a massive collection of extensions, you're much better off removing the alternator, whose lower bolt is the worst to remove. Then you can get to the two thermostat bolts. Lots of blind feeling around for this project.
 

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