Replace Thermostat?

Joined
Oct 27, 2006
Messages
281
Location
Illinois
My 2005 Envoy through a P0128 code a couple weeks ago which came back as a thermostat code. I reset the code with my Blu Drive reader and it hasnt come back since. I also have been watching the real time temp readings, the car warms up quick, heat is nice and hot and the temp gets to around 185, drops to 176 and stays consistent at 185, the dash gauge reads at the 210 level and since its really cold out, will drop if I am stopped idling and warm back up when driving. I also pulled the radiator cap when cold and there is no movement of the coolant which to me would presume the thermostat is working. Normally I would just replace the thermostat and be done but this car is a pain and requires removal of the alternator and other parts to get at the thermostat so I dont want to mess with it unless it really is a problem. Should I consider the code to be a 1 time thing and go with it or just take on the job and get it done? Thanks!
 
Joined
Oct 10, 2008
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Sunny Florida
I own a 2006 SRT8 which will occasionally throw a PO128 code when it gets cooler. It takes 3 slow warm ups to get the code set and usually 3 good ones will turn it off automatically. Looks like nothing to worry about to me...
 
Joined
Feb 5, 2008
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Atlanta, GA
My Malibu temp gauge stayed on cold one morning back in September. It showed a P0128 code. I later changed the thermostat and it was definitely bad, the inner rubber seal was gone.
 
Joined
Nov 20, 2006
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MA, Mittelfranken.de
It could be a coolant temp sensor or contact issue also, cold weather can wreak havoc with contacts and connections. If the sensor is easy to get at I would check for moisture or water ingress and clean the contacts with a contact cleaner and see how it goes. As it has good heat and the thermostat seems to be functional I doubt the thermostat has failed then started working again.
 

carguy996

Thread starter
Joined
Oct 27, 2006
Messages
281
Location
Illinois
I thought about the coolant temperature sensor, its in the same place as the Thermostat so its recommended to do both on this particular motor. Since its seems ok for now, ill let it go the rest of the winter and tackle this job when its a little warmer in the spring. This is mostly a winter car for us because of the 4wd and only used in the summer to pull a popup camper. Its starting to need some other repairs so it may get to the point where I just sell it since its starting to become a money pit!
 
Joined
Dec 31, 2017
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SE British Columbia, Canada
I’ve dispensed this advice in the past and it has helped some folks make the decision on the thermostat. Buy one of those infra red temperature guns with the laser pointer. Check the temps of the “hot” engine at the thermostat housing, radiator, and engine block. See how close this is to what your gauge is indicating. SF
 
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Joined
Jun 25, 2009
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Chicago, IL
I don't see any impending reason to change it immediately; but due to its age and this anomoly, I would put in on a list to replace sometime in the future when its convenience for you.
 
Joined
Nov 22, 2004
Messages
9,025
Location
Texas
The last thermostat I did on a trailblazer/Envoy I didn't follow the service manual that says remove generator. I just removed the wheel and inner fender liner and it's right there.
 
Joined
Apr 27, 2012
Messages
13,256
Location
MA
Last time I changed out a bad thermostat on a car that was running cold, the gas mileage improved afterwards. You just have to weight that with the difficultly of removing it and see if it's worth the extra gas mileage. I think it has to register cold several times before it trips a code so it's probably only a matter of time before it comes back.
 

carguy996

Thread starter
Joined
Oct 27, 2006
Messages
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Location
Illinois
Originally Posted By: GMBoy
The last thermostat I did on a trailblazer/Envoy I didn't follow the service manual that says remove generator. I just removed the wheel and inner fender liner and it's right there.
Thank you for that advice, Ill take a look into it!
 
Joined
Nov 22, 2004
Messages
9,025
Location
Texas
Originally Posted By: carguy996
Originally Posted By: GMBoy
The last thermostat I did on a trailblazer/Envoy I didn't follow the service manual that says remove generator. I just removed the wheel and inner fender liner and it's right there.
Thank you for that advice, Ill take a look into it!
You're welcome. It's about a 40 minute job this way. Remove the inner plastic wheel liner and it's wide open. Report back with your results.
 
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