Chevy Trailblazer Thermostat Replacement- P0128

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

Thank you for the tip/donation. 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.

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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.

Should you drain the coolant first?
 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.

Ky 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 2003 Chevy 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 SUPPOSE TO BE THERE.

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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.

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!

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 a 2003?

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.

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