Where is The Thermostat on Chevrolet Trailblazer

This article comes about by the seemingly mass confusion of where the thermostat is located on 2002 and up Chevrolet Trailblazer, GMC Envoy and similar body styles with the 4.2 liter inline six cylinder engine. I’ve seen many times, in person and online, people claiming the thermostat is on the right (passenger) side of the engine, inside a small housing that looks very similar to the V6 engines. This is NOT the case at all, and people that do not know what they are talking about can be misleading the regular guy trying to fix his vehicle.

Chevy Blazer

On earlier model vehicles, and even the S-series Blazer that was still out when the new body style Trailblazer came out, they had a V6 engine with the thermostat located right in the front, on top of the intake, just about dead center of the engine. When the inline six engine came out in 2002, the thermostat was re-located to the left (drivers) side of the engine, behind and a bit below of the alternator. The housing and hose that is now on the right side of the engine is just an inlet housing for the coolant. There is NO thermostat inside that housing on the Chevy Trailblazer.

Thermostat Pictures

As you can see in the provided pictures below, the thermostat is an integral part of the outlet housing, and is NOT serviced separately. You CANNOT just go to the parts store and get a thermostat in a box. This unit needs to be replaced as a whole. The repair can be made through accessing the bolts to the housing through the left front wheel well opening, or if you prefer, the belt, and alternator can be removed, but that is a bit more work.

I really hope this clears up any confusion you may have heard elsewhere. When it comes to a situation like this, you really need the correct information from someone who does this for a living!

Chevy Trailblazer thermostat housing assembly

This is the housing with included thermostat assembly located on the left side of the engine.

Do i need to purge cooling system on 2004 GMC Envoy after draining cooling system?

After refilling the radiator, let the truck run for a while with the cap off. It should bleed out any air easily that way. You may have to top off once the thermostat opens, but probably not that much. Once it’s hot, and circulating, put the radiator cap back on and you’ll be good to go. This also works on the Chevy Trailblazer.

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30 comments to Where is The Thermostat on Chevrolet Trailblazer

  • butch

    Great pictures Protech. I read on forums all the time people saying the thermostat is in the upper radiator hose- which it is not. Maybe now they will see this article and put the argument to rest ! That part is hard to replace. Antifreeze gets all over the place and one of the bolts is a a pain to remove.

  • Lissa

    You’re the graetset! JMHO

  • Anonymous

    Sorry to say but my 2004 has the thermostat located on passenger side of block near power sterring pump. Trying to figure out how to get out and find right part.

  • Wink1216

    If what you are saying is true and that the vortec inline 6 has no coolant temp. sensor then why does auto zone sell one ?Duralast/Coolant Temperature Sensor
    For your 2004 GMC Truck Envoy 4WD 4.2L SFI DOHC 6cyl
    Price: $29.99
    Please help before i spend my money

  • Wink1216

    the link that brought me here says there is no coolant temp. sensor see thermostat

  • Wink1216

    Quote 04 GMC envoy 4.2L can you tell me where is can find coolant below temperature sensor.

    There is no such sensor. I am guessing that you have a
    check engine light with code P0128- coolant temp below
    thermostat regulating temperature. This code is cause by
    either low coolant level, or more likely- a bad thermostat.

    • Phil

      Yes. If you read the sentence of the page your just posted, it says- where can I find the “Coolant Below Temperature Sensor”. There is no sensor called that. There is however a “Coolant Temperature Sensor”.

  • Wink1216

    Changed thermostat today . What a Mother %$%%$$ to get to! also changed belt while alt. was off thanks for the help. Peace out and Merry Christmas !

    • Norshamila

      This sounds like the the radiator may have had an air pocket and needs the coolant topped off. Also may need to replace the tiadaror cap as the system needs to be able to build pressure. Worse cause would be a blown head gasket but you would notice a loss of coolant in the tiadaror.

  • In a jam

    What tools do I need to replace my thermostat on a 2005 trailblazer trying to do it my self and any of you guy have a problem with a number 5 cylinder miss I replace spark plug,coil pack and rubber boot or spark plug wire and it still missing help

  • acdelco tech Lk.Limo Tavares,Florida

    Nice article and clarification on 4.2L I6 Trlblzr,GMC,Envoy;Several emails & mechs. have put articles for DIYs out there w/incorrect info…its not one text book proceedure fits all..appreciate those of us actually work on these vehicles putting accurate articles with pixs or diagrams as yours…keep em running brother!!!

  • needAbeer

    so I tried goig through the wheel well (had to buy an extension) but the top bolt is behind the alternator, how did others get to that bolt?

  • Sammy

    Use a universal joint or even better a wobble end extension. That will give you enough angle to get that bolt to remove the thermostat housing.

  • felix

    thank you,almost starting replacing the thermostat on the passenger side!

  • gladys

    i was putting a thermostat for a 1999 oldsmobile alero.and now i cant get the bolts to go all the way back in the housing cover where the thermostat go.help!

  • leonard

    i have a dtc 0f po128 07 tiail blazer

  • jtb1977

    Damn I wish I had seen this article before killing myself for THREE HOURS taking that stupid thing off on the passenger side!!! I was told by a “mechanic” that thats where it was…lol. thanks fir the great info!!

  • marty

    It’s about time somebody knows what they’re talking about. Thanks for the info.
    I have a 2003 TB and there is a lot of bad info on some sites.

  • marty

    I just completed the thermostat installation. Had to move the alternator though to get the top bold out.
    It was just too close to get in with the extension. Moving the alternator made it a lot easier to install the therm. housing
    and hose. It was a lot of work to get it done. But I’m glad I was able to do it.
    Thanks again for that good info.

  • Kedar

    Thanks pal for the reply..Sorry it took so long for this ,thanks the the TCCM Is on the Left side on top of´╗┐ the Heater vent on left side , i had to remove the 4 lower bolts and 2 bolts that held up the left Heater vent and was right there..Man i wedonr why they put such a part in this place ;} ??wish it was on the fire wall like other yr models Ughh any way thanks anyway..

  • Capt.Bill

    I am having over heat problems but h20 seems to be moving when thermostat opens. I look underneath engine and all is dry, no fluid on ground, but in the rear of blazer out of what looks like A C pipe there is water gatering on a sheet metal cross member. Does engine coolant run thru A C pipes or ?

  • Chubbo1965

    WE just changed the Thermostat on my 2055 Chevy trailblazer 6L 4.2 its on the right side behind
    the alternator.

  • jennifer

    i just changed my thermostat my self last saturday and now my temp gauge is saying the truck is running at 212 but actually running at 204 by the computer. the only thing is im not running hot. all the fluids are full no leaks or cracks. what could be the issue? im also thinking about changing the coolant temp sensor as well. any help PLEASE im supposed to be driving to south flordia for christmas and dont want to get stuck

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