How to Replace ECT Sensor and Thermostat 2004 Trailblazer 4.2 [Archive] - Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum

: How to Replace ECT Sensor and Thermostat 2004 Trailblazer 4.2


tspatterson
05-22-2010, 03:19 AM
First this is my first post, please forgive me if I use laymen terms whereas I am not a mechanic.

I got a DOCII P0128 Error 2 weeks ago, it cleared and re-popped about a week later.

For a 2004 Trailblazer 4.2 this code points to temperature problems related to the thermostat or ECT (Engine Coolant

Temperature) Sensor.

After researching what would be involved to fix this problem, I decided to replace the thermostat, sensor and

coolant all at once. In retrospect I am glad I did so.

Materials:
ECT (Engine Coolant Temperature) Sensor
Thermostat and assembly
2 gallons of PEAK Coolant

Tools:
Flathead Screwdriver
Pliers
3/8" Ratchet
8" extension
5/16 Socket
10mm Socket
15mm Socket
18mm Socket
18mm Any of these - (Crowfoot/Slotted Deep Socket/Butterfly/Offset)
18mm Box Wrench
15mm Box Wrench
Wire cutters
3 Gallon bucket/container

Step 1
Put the vehicle on ramps or a jack

Step 2
Disconnect the battery cables using the 5/16 socket and 3/8" ratchet. (I start with the positive and then the

negative)

Step 3
Take your pliers and bucket go under the vehicle.
With your head towards the back of the vehicle locate the large rubber hose connected to the bottom of the radiator,

this will be on your left.
Take your pliers and slide the pressure collar up the rubber hose about 3-4 inches.
Place your bucket under this junction.
Slowly back the rubber hose off of the radiator connection.
When a controlled steady stream of coolant comes out, go to the top of the engine.
Remove the radiator cap and the coolant reservoir cap, this will speed up the coolant drainage. (for a 2004

trailblazer 4.2 total drainage time was about 15 minutes.
Place the rubber hose back onto the bottom of the radiator and move the pressure collar back into position.
Go to the top of the engine.

Step 4
Take you 3/8 ratchet and insert it into the square plug on the tensioner pulley. (If you are standing at the front

of the vehicle the tensioner pulley will be on your far left the second pulley down.)
Set your ratchet for tightening, and turn the ratchet clockwise. (You can use a small piece of pipe over the ratchet

handle for leverage, however it may be impeded by a large rubber hose exiting the top of the radiator.)
Remove the serpentine belt from the alternator pulley. (If you are standing at the front of the vehicle the alternator

pulley is the topmost pulley on your far right in a 2004 trailblazer 4.2)
Slowly release the tensioner pulley once you've removed the belt from the alternator pulley.

Step 5
Take a 15mm socket and 3/8" ratchet or 15mm box wrench and remove the 2 diagonal bolts on the front of the large

black plate that partially covers the top front of the alternator.
Add the extension to your ratchet and just behind the black plate remove the third 15mm bolt from the black plate.
Switch to the 10mm socket and remove the 4th and smaller bolt on the front of the black plate.
Completely remove the black plate from the engine.

Step 6
Switch to the 15mm socket and 3/8" ratchet and remove the top 2 bolts that hold the alternator to the engine.
Remove the 3rd and final bolt from the bottom of the alternator. (There is a hose that blocks the bolt from coming

strait out, but once its loose, you can turn the alternator clockwise [if looking top-down on the alternator] and

work the bolt out.)
Rotate the alternator from back to front.
Remove the plug clip from the alternator.
Pull back the rubber boot covering the large wire connected to the alternator.
Use the 10mm socket and 3/8" ratchet and remove the nut and disconnect the wire from the alternator.
Rotate the alternator right to left and remove if from the engine cavity.

Step 7
Use the 10mm Socket, the extension, and the 3/8" ratchet and remove the 2 bolts connecting the thermostat assembly

and large rubber hose connected to the engine block just behind the alternator you just removed.
(A little more coolant will drain out)
Take the pliers and back the pressure collar up the large rubber hose you just disconnect from the engine block and

back the it up the hose about 3-4 inches.
Remove the thermostat assembly from the rubber hose.
Check to make sure the small rubber gasket (looks like a rubber band) is either out of the engine block connection

or with the thermostat assembly.
Replace the thermostat assembly with the new one into the large rubber hose.
Insert the assembly into the hose and stop at the ringlike bump on the assembly.
If you are looking at the assembly and hose from the top-down, the apex of the metal assembly angle points toward

the back of the vehicle.
Use your pliers and back the pressure collar back into place at the end of the rubber hose.
Take the 10mm socket, extension and 3/8" ratchet and start the bolt closest to the front of the vehicle back into

the engine block.
When you have about 1/4 left to tighten, take the new thermostat gasket and put it into place in the groove of the

thermostat assembly.
Once in place push the assembly up to the engine block being very careful to keep the gasket in place.
Tighten down the bolt the rest of the way to near finish tightness. (You need to be able to rotate the hose and

thermostat assembly a little to align the back bolt but not loose enough to allow the gasket to dislodge)
Replace the second 10mm bolt into the lower-back hole in the thermostat assembly at the engine block.
Tighten both bolts the rest of the way.

Step 8
Take your flathead screwdriver and at the junction plug of the engine and the ECT sensor remove the 2 grey pins

holding the plugs together.
Disconnect the ECT Sensor from the electrical system.
(Unless you have one of the specialty sockets listed above) Cut the pigtail off just behind the little rubber cup at

the back of the old ECT sensor.
Use a 18mm socket to remove the old ECT sensor from the engine block.
Be sure to check the threads on the old ECT sensor versus the new ECT sensor and make sure they match, I had an issue

where one brand from Advanced Auto listed the sensor for my vehicle but the threads did not match.
The sensor comes pre-coated with threadlock.

YOU WILL NEED ONE OF THE SPECIALTY SOCKETS LISTED ABOVE TO CONTINUE!!!!

Take one of the 18mm specialty sockets listed above and screw in the new ECT sensor. You should expect some

resistance due to the threadlock.
Tighten the new ECT sensor very tightly all the way to the metal washer.
Clip the ECT back into the electrical system and replace the 2 grey clips back into the clip assembly.

Step 9
Reverse the instructions for alternator, black plate, and serpentine belt removal.

Step 10
Fill the radiator and reservoir (to the cold engine marker) with new coolant. For my 2004 trailblazer 4.2 it took

about 3 gallons of coolant/water mix to fill everything. Replace both caps respectively.

Step 11
Reconnect the battery cables.

Your 2004 Trailblazer should run right at 210 degrees with only 1 to 1/2 tick mark variance. I have noticed a slight

improvement in fuel efficiency and a decrease in the "diesel" sound in the engine with starting cold and idling.

I hope this helps.

If you have any questions please feel free to email me at: timothy.s.patterson@gmail.com

the roadie
05-22-2010, 10:14 AM
Great first post! Thanks. Your short description of the third alternator bolt is a symphony of understated frustration to those of us who have been there. ;)

If there were pics this would be a dream article for searchers to find.

elementkidd
05-22-2010, 10:40 AM
Haha this is great i just did this 2 days ago! Step by step, i should of looked on here first and man is that third bolt really hard to get off... Who would design that so when a bolt is tightened it rests on a A/C hose?:hissy:

tspatterson
05-22-2010, 12:10 PM
Great first post! Thanks. Your short description of the third alternator bolt is a symphony of understated frustration to those of us who have been there. ;)

If there were pics this would be a dream article for searchers to find.

Yes, I apologize about the absence of photos, I was afraid I might in fact be photographing a disaster if I had finished the repair and all did not go well. :) However I am working on posting schematics and 3rd party images to help out.

A pie chart for time allotment on removing and replacing the infamous 3rd bolt on the alternator might be handy, I have an image in my head of something like pac-man chasing a stick. To the engineers at Chevy, Thank You, Thank you so much, and from us in the nursing community be happy we don't have to remove your lungs to get your gall bladder....

luarg2
06-19-2010, 06:22 PM
The key to removing that infamous lower alt bolt is a 5inch 15mm wrench. It took some old school elbow grease but it came out with no problems.

floridajetsfan
08-25-2010, 03:49 PM
Timothy,
Thank you so much for the incredibly detailed instructions! This is exactly the job I'm doing on my 03 Trailblazer next weekend and have been getting conflicting accounts on the "degree of difficulty" from the local Chevy dealership. I'll let you know how I manage!

Thanks,
Mike :thx

FunkZ
08-29-2010, 04:23 PM
Did the thermo on mine today, about 2-2 hours. The bottom alt bolt is inconvenient to get to but I just bent that AC line out of the way and got it with an open wrench. Compared to removing the alt on my GA V6 where the bottom bolt backs out into the firewall, or some of the other crap like taking off the engine mount to change the belt or remove the PS pump, the TB didn't seem so bad. I went ahead and did the plugs too, that was fun getting the rear two, had to climb into the engine bay. Just turned over 100k so I'm sure I'll be replacing a bunch of other stuff soon.

joric
09-18-2010, 11:24 AM
:thx So The serv engine light comes on and I scan to see.. and its P0128.. Found this info most helpful.. thanx man..

Lawnranger
12-14-2010, 06:31 PM
Got error code P0128 today so I went ahead and changed the thermostat and the ECT sensor. Found this information to be very useful. I didn't have any problem with the 3rd alternator bolt. I just loosened it a little and I was able to use my bare hand the rest of the way.

Now its time to focus on changing the motor mounts and cleaning the throttle body :crazy:

Thanks again for the info :thumbsup:

Loverotties
12-14-2010, 11:41 PM
If your motor mounts are bad(weird that both are bad)you should of did it with the alt. out of the way.

tspatterson
12-15-2010, 12:06 AM
I've done a little reading as to why my Chevy 2004 4.2 V6 Trailblazer sounds like a small diesel engine. I too have heard that the engine mounts could be shot, I've cleaned the intake several times (Can cause the truck to stall or act "weird" if you turn in the AC the first few times you run the truck after doing this), I've replaced the spark plugs with high dollar plugs, and the wire set/boots. I also put in high mileage oil (non-synthetic) without a change in how it runs. I'm at a loss really, I'm not a mechanic but a guess would be something larger is the problem, my fear is that its a piston issue, even though Chevy swears that this "diesel" sound is normal for the truck, my belief is that there's a difference between a cool purr and poor mechanics. :)

tspatterson
12-15-2010, 12:18 AM
I recently spoke with another gentleman who had worked through my steps and after completing the job noticed a clicking sound for the first few minutes the truck was running. This sound would go away and the truck ran normally. He noticed that the sound was coming from the rear, right quarter panel. He did not mention and issue with his AC/heating, however I knew what the problem was. On the occasion after disconnecting the battery for long periods of time, say about 20 minutes. When the battery is re-connected the truck goes through a climate control reset/recalibration. Most people complain of the clicking followed by either a constant running climate control, only no heat, only no cool and can only effect the rear climate control system (if the sound is coming from the rear of the truck).

This issue is a bad or dead Doorman Valve. This mixes the air temp for your rear climate control, its located in your rear right quarter panel. You can take it out, it looks like a rounded end computer hard drive. Tap it gently a few times, rub the pin connections with an eraser, (if you ever had an original nintendo you'll know how to do this) and plug it back in. You might buy yourself about 3 -5 months before it completely dies, if that process works at all. You can disconnect it completely, if you never use the rear climate control then it shouldn't be a big worry, disconnecting it will stop the clicking. If you need to replace it, I wouldn't suggest getting a new one from the parts store whereas its about $120 - $140, go with used or on the internet.

Loverotties
12-15-2010, 01:45 AM
I've done a little reading as to why my Chevy 2004 4.2 V6 Trailblazer sounds like a small diesel engine. I too have heard that the engine mounts could be shot, I've cleaned the intake several times (Can cause the truck to stall or act "weird" if you turn in the AC the first few times you run the truck after doing this), I've replaced the spark plugs with high dollar plugs, and the wire set/boots. I also put in high mileage oil (non-synthetic) without a change in how it runs. I'm at a loss really, I'm not a mechanic but a guess would be something larger is the problem, my fear is that its a piston issue, even though Chevy swears that this "diesel" sound is normal for the truck, my belief is that there's a difference between a cool purr and poor mechanics. :)

Straight 6 or V8 no V6.

Lawnranger
12-15-2010, 07:24 AM
If your motor mounts are bad(weird that both are bad)you should of did it with the alt. out of the way.

I would have liked to have done the mounts while I was doing the thermostat, but I didn't have time before heading out to my 2nd job to get them done. I will definitely be taking the alternator out when I finally have the time to change the mounts ( maybe Friday).

I Don't think that both mounts are bad, but I figured why only do one. From what I have read, even a good mount will get compressed and be a little shorter than a new/original one.

weebal
02-20-2012, 03:34 AM
thanks for this post it came in very very handy!!!!! :tiphat:thx:thx:thx:thumbsup::hail:

SumEMT
07-17-2013, 09:51 AM
:thxReplaced my thermostat the other day (stuck open by maybe a 1mm...2 at most, but hey, that SEL will keep me from passing emissions in WI)!

I have next to no mechanical experience at all, but between your post and some YouTube help, I was able to get her done in about 2 hours. Overall, it was pretty simple for a dummy, but yeah, bolt location made some parts a pain the rear!

Some recommendations; For both the bottom thermostat bolt and the bottom alternator bolt, jack the truck up a bit, you don't have to get the wheel off the ground, but make some clearance in the between the tire and the wheel well. You should see the bottom thermostat bolt easily! The bottom alternator bolt isn't as easy to see, but move a little of the black plastic out of the way from the wheel well and you can access it! Then get yourself a looooong socket extension (I bought a kit at Harbor Freight with 4/5 different sized extensions for about $8). I connected all the extensions together which gave me about 14-16 inches of extra socket! I can't stress how much easier it made the job! My bloodied/skinned/bruised knuckles would have appreciated knowing this sooner!