NEED HELP 05 Trailblazer 4.2 ECT vs Thermostat vs Scanner

wbrockstar

Original poster
Member
Feb 6, 2019
5
Nashville,Tn
My temp gauge has been fluctuating for a few months now,normal readings 90% of the time.It normally stays at the 210° mark,but sometimes it'll drop down to the next tall hash mark,hang around there for 5+ minutes then go back up to normal again.A couple times the gauge would drop down to the key off position then it would go back up to the 210° position and remain there for the remainder of the drive.The cel has popped up (P0128) maybe twice in the last six months & after pulling codes I would clear it.When it popped up a few days ago,I checked the ect with my scanner over the next 3 days when I got home from work.The scanner showed the ect as being 25° one time and 31° the other two times.Now,Ive gotta assume that even if the tstat was stuck open and coolant never had time to remain inside the engine long enough to get it up to 210° or anywhere close,it seems almost impossible to me that the coolant temp could actually be this low after the vehicle had just been driven on the interstate at 75mph for an hour.Does this excessively low reading pretty much verify the ect sensor is defective vs the tstat?? I bought an AC Delco tstat months ago and haven't changed it yet,but Im trying to figure out if its the ect sensor instead?? Im gonna replace the thermostat regardless,but if its caused by the ect sensor instead,I need to buy that beforehand since Ive read its best to replace both of these parts at the same time due to their pita location.Can the ect sensor be tested with a multi-meter,and if so,is there a chart available with the temp/ohms/
voltage specs to use for testing and if it needs to be replaced,is AC Delco the only brand worth buying?? If the ect sensor is outputting a value equivalent to 140°,but coolant temp is actually at 210°, does the pcm rely solely on what the ect sensor is telling it in order to control the fuel trim and does the fact that the coolant running through the engine is really at 210° not have any influence at all on how the engine runs??I know the cooler the ect is,the more fuel the pcm will add.So if the ect is really running at 31°,shouldn't the fuel trim be so rich that driveability issues would be present or a no start condition would be caused?? When a ect sensor went out on the Mustangs Ive got,the sensor would always register way too cold of a temp and a engine flood condition would ensue and pressing the gas pedal down 1/4 of the way or flooring the pedal during crank mode would be the only way I could get it to start.The 25-31° ect reading seems like it would trigger the same symptoms, unless things work differently on the TB without a maf sensor. There were two seperate times though,where it acted like it was starving for fuel or getting too much during a hot restart,but once I shut it off and restarted it,it ran just fine.
Hopefully you can give me some answers to these questions.I'll be waiting on your responses. Thanks....
 
Last edited:

Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
The ECT sensor is the only one it has to read engine temps. If the sensor is off or the tstat is stuck open, it usually throws that code. I'd just replace both, which we usually recommend anyway because of their proximity and difficulty to get to. Reading that low, my money is on the sensor.
 
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wbrockstar

Original poster
Member
Feb 6, 2019
5
Nashville,Tn
Yeah I was feeling the sensor too.Every thread Ive read on this type of issue showed members reading temps of 170,180,185°, etc,but I never saw any posts with 25-35° readings.Being that the TB is used and I have no clue as to whats been replaced,it could be the original sensor or a newer one.I just didn't want to buy a part if it wasnt needed.
 

wbrockstar

Original poster
Member
Feb 6, 2019
5
Nashville,Tn
Any suggestions on whether its easier to go through the fenderwell,from above or from below and what specific item(s) is in the way when trying to access the bottom bolt everyone complains about?? Is it the ac line,part of the block or what?? Im not at all looking forward to it....
 

Blckshdw

Moderator
Nov 20, 2011
10,665
Tampa Bay Area, FL
The alternator is what's in the way. if you decide to go through the driver's side wheel well, you'll need some extensions to reach it. It's difficult to see, as there's not much room. If you're comfortable doing it, removing the top 2 bolts of the alternator, and rotating it forward a few degrees will give you the clearance to go at it from the top. Little more time consuming, procedure wise, than the wheel well, but much more straight forward. :twocents:
 
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TJBaker57

Member
Aug 16, 2015
2,897
Colorado
Before replacing a sensor (any sensor) I would throroughly verify the wiring of said sensor. On my 4.2 temp sensor I found a bad connector and remedied that. Granted, my TrailBlazer came to me with a hacked up wiring harness that even now I have no idea what could scenario led to what I see in my harness.
 

wbrockstar

Original poster
Member
Feb 6, 2019
5
Nashville,Tn
Ok I'll try using that method,since that'll allow the alternator to move out of the way vs being removed completely from the vehicle.Another thread had mentioned moving/bending the ac line out of the way too.

I'll check the condition of the harness/plug also.I assume AC Delco is the only reliable brand to buy,correct??
 

Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON

wbrockstar

Original poster
Member
Feb 6, 2019
5
Nashville,Tn

Ok thanks.Out of curiosity,what is it specifically that causes aftermarket sensors to have a high failure rate vs oem sensors?
Do their manufacturers use cheaper materials or what?? Is the ect sensor tested the same way regardless of year/make/ model and are the ohms/voltage chart values pretty much the same too??
 

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