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Discussion Starter #1
Lately my '06 I6 (95K miles) has been regularly throwing a P0128, and it does seem to be running a bit cooler than normal, so I need a new thermostat. I know these forums are full of great how-tos, but I'll probably just pay a pro eventually.

My question is, do I really care about this right now? I live in Iowa where I might (will) be in need of heat in a few months, but it doesn't seem like it should otherwise be cause for concern, especially when we're regularly flirting with 100 degree heat indices.

FWIW, I recently had a coolant flush and fill, at which time it was discovered that the water pump needed to be replaced. This thermostat problem existed before that work was done, and if I'd been thinking I would have had the thermostat replaced then.


Thanks!
 

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2002 chevy trailblazer_ls
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These trucks are designed to run pretty hot. If the temp doesn't come all the way up, you'll end up burning more fuel. In the short term it's not going to hurt anything.
 

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2004 chevy trailblazer_ls
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I've also read that a cold/cool running engine will prematurely kill the cat. I can't personally vouch for the validity of that, though.
 

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2003 gmc envoy_slt
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An engine that does not get up to it's designed operating temperature will use more fuel, and will have a higher risk of sludge as the condensation in the engine is not getting heated enough to evaporate.
 

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2002 chevy trailblazer_ls
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An engine that does not get up to it's designed operating temperature will use more fuel, and will have a higher risk of sludge as the condensation in the engine is not getting heated enough to evaporate.
True that. Even short driving distance that don't allow the engine to get fully hot will not burn off the condensation.
 

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Discussion Starter #6
Thanks folks, I guess I'll get it fixed sooner rather than later.

Driving home from work today, I actually used the AC, which I rarely do, and the temp gauge showed a normal temp, 210°. At the same time, Dash Command displayed a temp around 190°. Which is right? Could both be wrong?
 

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2004 gmc envoy_slt
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This engine is equipped with a 195* thermostat and the gauge will normally indicate just short of 210* when that temp is reached. These gauges are not that accurate.

Your P0128 could is being generated because the engine coolant temperature is not reaching operating temp in a prescribed time period. It is a sign of a lazy Tstat. Replace it. In the long term, if this condition remains, the cat will eventually become clogged from a rich fuel mixture.
 

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2003 gmc envoy_slt
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I'd go with the Dash Command reading. It gets it's info directly from the PCM.
The dash gauge is displaying what the PCM is wanting the driver to see. Manufacturers have been programming the temp gauges to read steady so as to not alarm the driver if the temp goes up or down a bit.
 

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Discussion Starter #9
Manufacturers have been programming the temp gauges to read steady so as to not alarm the driver if the temp goes up or down a bit.
Well that's annoying. I'd feel better seeing it move up and down. Failure to move would be the first sign that the thermostat is stuck open.
 

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2008 isuzu ascender
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I'd go with the Dash Command reading. It gets it's info directly from the PCM.
The dash gauge is displaying what the PCM is wanting the driver to see. Manufacturers have been programming the temp gauges to read steady so as to not alarm the driver if the temp goes up or down a bit.
That is correct believe your device that is plugged into the obd2 socket the info comes right from the computer . It will plug the cat eventually , maybe not during the summer but once its gets cold .
 

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2003 chevy trailblazer_lt
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I ran for years with a stuck open thermostat, here in warm Kali it was nice to have the truck run cooler, but the smog man caught up with me I couldn't clear the code anymore right before smogging every other year so I had to change it. AND today, I replaced the cat which was about 50% clogged. WHAT A DIFFERENCE. I have my old Trailblazer back! At 230k miles this truck runs almost like new again. If your TB falls on its face when you gas it, or wont rev past 4k, your cat is done. New aftermarket cat, (weld in) was $174, what a deal dont buy GM. I used the how to's on this site to change out the thermostat, its a PITA if you aren't handy with a wrench or enjoy getting your hands cut up take it to a mechanic, JMO.
 
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