2003 Chevrolet Trailblazer P0128.
4.2 Inline 6 cylinder
Service engine soon codes  P0128.

Question = My service engine light is on. The dealer said the code is P0128 "Coolant Thermostat"
(Coolant temperature below thermostat regulating sensor).
My temperature gauge reads okay and the vehicle runs good. Is it difficult to change this myself?
(I do have some mechanical ability) If so, can you provide me instruction?
Answer: Thank you for the tip.
Yes, this is a common cause for P0128.
They are not too difficult, but a little tricky.
The thermostat is the actual thermostat and the housing as one unit.

Best way is like this...
Remove drivers side wheel. It is easiest to get through the wheel well.

Use a 10mm swivel socket or a regular socket on a universal joint. Remove the 2 bolts on the housing.
Leave the hose connected to the housing.

Grab the bolts before they fall.

Then go on top and remove a 10mm bolt that holds the large A/C line on the top. This gives a little
more room for next step.

Pull the housing / hose up so can work on it from the top. Much easier than trying to do everything
from the wheel well.

With a marker, mark the hose and housing to index the housing. The hose can be very stiff, and doing
this will ensure that when you put the hose and housing back down to the side of the engine, it will line
up just perfect.

Now remove the hose clamp and hose from housing. Install the new housing in the same position
using the mark on the hose. Now push hose back down, and it will line up just perfect to install the
bolts through the wheel well. Be sure to scrape the matting surface on the block nice and clean.

I will send a pic of its location in a few minutes.
Problem with my 2002 Envoy 4.2 inline 6 code P1481.
Replaced the cam actuator solenoid.
The code P1481,what kind of problem does it cause .My engine some time idles at 600 rpm,but some
times it jumps to 900 rpm. Sometimes it shut off when you come to a idle.
The code is for the electrical coolant fan circuit. The powertrain control module (PCM) uses the cooling
fan speed signal in order to determine the actual fan speed in relation to the desired fan speed. The PCM
uses the cooling fan speed signal in order to reduce the cooling fan noise and in order to maintain the
powertrain cooling requirements.
You may have a damaged harness for the fan per a service bulletin, or a faulty fan. This would not cause
it to stall or idle bad.
This stalling sounds like a common problem of carbon in the throttle. A good cleaning is all that is
needed. You can watch a video here on how to clean it out.
thank you for the info.
Should you drain the coolant first?
Tech: Yes you drain the coolant first, but there will still be
a lot left over that will spill pout when the thermostat is
removed, so just let it all come out that way. It does make
quite the mess though.
He asked about a 6 cyl sounds like your telling him
how to change one on an 8 cyl.
Nope. This procedure is for the straight 6 cylinder
engine. The thermostat on the V8 is on the front of the
engine, on the water pump housing.
my inline 6 trailblazer the top hose to thermostat is
on passenger side and i cant get a socket or
wrench on it even if i take the tire off the power
steering in the way
The thermostat on a Trailblazer Inline 6 cyl is on the
drivers side, lower hose, behind alternator, not in the
upper hose.
The thermostat on my 03 trailblazer in on the
upper passenger side 4.2 i6
If it is actually there, someone put it in the wrong spot
because it is not on the passenger side top. If you are
referring to the upper radiator hose, IT IS NOT
Mine is there on top as well. Never touched it, it
was there from the factory.
You guys are nuts! The only way the thermostat is on top (Upper radiator hose) is if this is
a V6 in the old body style Blazer(some where also called Trailblazer). ANY STRAIGHT 6 cyl.
Engine- Chevy Trailblazer, GMC Envoy-  the thermostat is on driver side of engine, middle
way down, behind alternator. Just because there is what looks like a thermostat housing on
the top of the STRAIGHT 6 CYL engine, does not mean there is a thermostat in there !
Well I'm not nuts, and i have a 04 trailblazer, Ive been
working on cars for a long time and i pulled a thermostat off
my inline 6 4.2 from the passenger side top of the motor.
If there was a thermostat in there, you now have two thermostats on the vehicle. Take a
look at the radiator hose on the driver's side of the vehicle. It winds its way back behind the
alternator to the THERMOSTAT HOUSING. The thermostat is ONLY replaceable as a unit
that includes a NEW HOUSING. You can't even buy just the thermostat for the INLINE SIX
CYLINDER, like you can for the V6 engine. If you have such doubts, please remove that
housing behind the alternator, and you will find a THERMOSTAT!!!! I've been a technician
at a Chevy dealer for 15 years, and unless you're severely confused as to what type of
vehicle we're talking about here, there is NO thermostat in the passenger side outlet
housing. And if there is, it was put there by a retard!
Thank you for the steps to replace the thermostat.
It went well following your advice.
In getting ready to change my thermostat on 03 trailblazer inline six. It does look like a
housing on upper passenger side but the new thermostat/housing matches the driver side
middle as previously stated. I would have to agree that it is on the drivers side mid way
down the engine from the lower hose. Why cant gm put these where easily accessible like
older engines?? grrrrr
Yes, indeed it is on the drivers side. Pain in the
kazoo to get the bolts lined up just right as to not
cross thread them. Thanks for the info.
I am going to do one on my 03 and what baffles me is are not the thermostats usually on
the upper radiator hose allowing the coolant to filter down through the radiator?....Does the
inline 6 in these coolant flow the other way?
Yes, the cooling system in your Trailblazer is a 'reverse
flow' style system. Most newer GM vehicles are all this
way now. They are actually a little better design and cool
more efficiently. It does make it harder to change the
thermostat, though, so just take your time and make sure
you do it right.
i have a head ache and stomach hurts from
reading u guys talking about thermostats
location!!!  I think the thermostat is in the rear
passenger side tire!!
aha it is def in the driver side behind the wheel well
definitely!!!!! I like how people like to debate on
things they have no clue about.
I like the expert let's give it up for him he knows
WTF he is talking about and not these wannabe
shade trees LOL.
This explains why every time I ordered a T-stat I got the wrong part. I expected it to be at
the top, never even looked at the "return" hose. I guess I've been out of that
end of the field too long. Time to get out of manufacturing and back to turning wrenches
instead, more satisfaction that way!
CarMD has this code as a $70 sensor. If the temp
is actually low with an external tester, then can it
mean the actual problem is the T-stat?
Nine times out of ten the problem is the thermostat when
you get a P0128 code. Rarely is it the engine coolant
temp sensor, that's what CarMD is probably telling you. If
it were me doing the job, I'd put a thermostat on it.
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I have a 2004 Trailblazer and got the same code and have done everything BUT put a new
thermostat in it which I am going to do tomorrow with a friend. Can you tell me what size
wrenches and tools I'll need to do it? Is the thermostat in the same location on an 04 as it is
on an 03?
how do you get the gasket to stay on when
changing the thermostat?
You can add a little gasket adhesive if necessary, but
don't over do it. It's best to not use anything, sometimes
you can thread one of the bolts through the housing and
gasket together, and as you start the bolt in the block it
should stay attached.
All this was very helpful thank you. Gonna go change the thermostat later today and hope I
don't get frustrated doing it. Best to take your time I guess.
Haha I just tried to change mine on a 04 trailblazer and was doing it from the top pass side.
Then I was looking at the part I bought and was like there is no way this well fit here! Thank
all u guys I didn't even bother to think to check the driver side. U saved me a lot of time
these website are very helpful to the common person who otherwise could not afford to pay
outrageous prices so thanks to everyone. From Las Vegas.
My friend and I are replacing the thermostat on a 2003 trailblazer and the tech at Carquest
wrongly told us that the thermostat was on the passenger side; we worked all day for
nothing. He also gave us a thermostat without the housing. If we had known about this
website ahead of time we wouldn't have had an entire day of wasted effort that came to
nothing. Thank you so much for your help in this forum!!
People need to keep in mind that the v8 models have a different system then the i6 models
and the thermostat I believe on the v8 is a standard thermostat not the housing unit type.  I
have the I6 and mine is the housing on the drivers side.  I am not sure if the v8 is the upper
passenger side where the standard thermostat goes in however working on different cars
over the years this would tend to make sense and be very possible if it was still running off
the old type of cooling system design.  Bottom line, someone could think they have the 6 I
guess when they actually have the v8 by accident.  JMHO for what it is worth as a seasoned
backyard mechanic.=-)
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Chevy Trailblazer Thermostat Replacement- P0128