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
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.
ECT (Engine Coolant Temperature) Sensor
Thermostat and assembly
2 gallons of PEAK Coolant
18mm Any of these - (Crowfoot/Slotted Deep Socket/Butterfly/Offset)
18mm Box Wrench
15mm Box Wrench
3 Gallon bucket/container
Put the vehicle on ramps or a jack
Disconnect the battery cables using the 5/16 socket and 3/8" ratchet. (I start with the positive and then the
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.
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.
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.
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.
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
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.
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.
Reverse the instructions for alternator, black plate, and serpentine belt removal.
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.
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: firstname.lastname@example.org