I am experiencing the same issue with my 2017 Acadia, which has 168,000 KM and is an SLE model. I checked the error using OBD2. After clearing the error, I monitored the coolant sensor, and it was working fine without any issues. However, after two to three days, the same issue reoccurred with the same error code. I cleared the error again, but it reappeared after a couple of days.
I am confident that the thermostat is working fine, even when the issue appears. I suspect it's an electrical problem, possibly related to battery voltage or loose cables. Sometimes

other error codes appear.
The main error code is


P0128 [0x0128]
Coolant thermostat - coolant temperature below thermostat regulating temperature
Status: Pending
----------------------------

Other errors sometimes appears

------****
Car Scanner ELM OBD2
Version: 1.101.9/401019/GP
DTC report
Connection profile: GMC OBD-II / EOBDDate: 12/5/2023 12:22:42 PM
VIN: 1GKFN8LS0HZ291604
============================
OBD-IIOBD-II
DTCs: 7
----------------------------
P0700 [0x0700]
Transmission control system, MIL request - circuit malfunction
Status: Confirmed
----------------------------
U0100 [0xC100]
Data bus, engine control module (ECM) A - no communication
Lost Communication With Engine Control Module Invalid Data; Lost Communication With Engine Control Module Message Counter Incorrect; Lost Communication With Engine Control Module Bus Signal Checksum Error
Status: Confirmed
----------------------------
P0128 [0x0128]
Coolant thermostat - coolant temperature below thermostat regulating temperature
Status: Pending
----------------------------
U0101 [0xC101]
Data bus, transmission control module (TCM) - no communication
Lost Communication With Transmission Control Module Invalid Data
Status: Pending
----------------------------
U0140 [0xC140]
Data bus, body control module (BCM) - no communication
Lost Communication With Body Control Module Performance - Signal Invalid; Lost Communication With Body Control Module Invalid Data; Lost Communication With Body Control Module Message Counter Incorrect; Lost Communication With Body Control Module Bus Signal Checksum Error; Data bus, body control module (BCM) - no communication; Data bus, body control module (BCM) - no communication
General Electrical Failures [0x00]
Status: GM failure record
----------------------------
U0131 [0xC131]
Data bus, power steering control module - no communication
Lost Communication With Power Steering Control Module Invalid Data
General Electrical Failures [0x00]
Status: GM failure record
----------------------------
U0121 [0xC121]
Data bus, anti-lock brake system (ABS) control module - no communication
Lost Communication With Electronic Brake Control Module Low Voltage; Lost Communication With Electronic Brake Control Module Invalid Data; Lost Communication With Electronic Brake Control Module Message Counter Incorrect; Lost Communication With Electronic Brake Control Module Bus Signal Checksum Error
General Electrical Failures [0x00]
Status: GM failure record

 

<snip>
I am confident that the thermostat is working fine
<snip>

How can you have a P0128 and be confident the thermostat is working fine? It's a very common failure.

 

I have a 2019 Acadia Denali. I have both the shift to park issue and now the thermostat issue. I reset everything by pulling the negative cable for about 5 minutes then reattached. Problem went away for about 2 weeks and when I went to use my cruise control, it immediately started the temp gauge falling, etc. Reset the battery again and went about a month and went to use the cruise control again and it started the problem again. I have been reading that along with the temp gauge going to cold, fan blowing continuously like it running hot, and other oddities, all I get is the a/c quits working and temp gauge quits working. I don’t get the fan issues and other things mentioned in previous posts. Going to try and not use the cruise control and see how long it lasts this time. Starting to wonder if it may be battery related? Only have 46,000 miles on it.

 

Did you happen to get it fixed?

 

I felt this same way as everyone- it must be a software, sensor or computer problem. The failure mode is not traditionally one that occurs with a bad thermostat- it usually gets stuck open or closed. Disconnecting the battery or resetting the computer always fixed it- for a random amount of time- a few days to a few weeks.
I bit the bullet and changed the thermostat, coolant temperature sensor and sensor harness end.
No problems now for two months. Money and time well spent.

 

its not the thermostat, neither the sensor or fan assembly. sent to pepboys under warranty they did the whole job for about 1600 problem was fixed for the 3 months and it came back up. its sometthing with the computer that no one seems to fox because the claim that the have to"RE-DO the whole assembly" complete bs. only thing wotking right now is resetting the battery by connecting/disconnecting neg termina

 

I am starting to agree that it is a computer related problem. Last week i was traveling and somehow it reset itself to where the A/C started back working and the temp gauge. So far it hasn't gone back off again.

 

Did you figure out what is was? M 2018 Acadia just did this same thing starting last night.

 

Read the whole thread. It's a failing engine coolant thermostat.

OR .. . read post #62 above if it's not too much trouble. . . .

 

Well here's a new twist that maybe unrelated but it is a strange occurrence
Whenever I'm traveling in a certain area in our town almost every time the dumb thing throws the code and the coolant temperature goes to zero and I have to reset it by pulling the lower harness off and putting it back on. Now I'm starting to wonder if OnStar or some random signal that OnStar is picking up is throwing that code! Just in the preliminaries but I'm going to start wrapping aluminum foil around my OnStar antenna to see if I can get rid of the signal and communication with OnStar altogether to see if it ever does it. Going to be a strange experiment but I'm going to try it just to see. Anyone else have any wild thoughts on that?

 

I've had this problem since last August. I bought thermostat and sensor, had it replaced. Error still happening. Bought new battery, still not fixed. Found different mechanic, said computer needs hard reset and to replace thermostat and sensor again. I'm not happy, GM had technical alert on this issue.

 

I have a 21 Acadia SLE and this has been and off and on issue. Seems when I disconnect my battery and wait it resets and sometimes it comes back in a week and sometimes it’s months. Took it to the dealership and they wanted $172 to hook up to the computer. I told them to take a flying leap. Definitely should be a recall with how many I’m reading have this problem.

 

Well. . . not a recall, but perhaps a "special program" to remedy it. Haven't seen too many 2021 with the thermostat issue.
Perhaps yours is a different issue which is why the dealer needs to hook up a diagnostic computer to see if there are any failure codes.
There is more than one component that can give this symptom and you do not state exactly what messages and operational conditions are affected by your particular issue.

 

I have a 2021 Acadia AT4 and the engine light came on today and the temperature gauge quit working. I stopped by an O’Riley’s in Wisconsin and the code says I have an a faulty temp gauge that’s stuck open. I drove about 150 to 200 miles further and the engine did not overheat and drove fine. I leave from Rockton, IL for home on Thursday and have another 750 miles to travel back to Alabama. I’m temped to drive home and take it to my car repair shop. Tomorrow I’ll unhook the battery and after about 10 minutes I’ll hook it back up and see if it resets. I hope it does.

 

Did you make it back ok?

Sorry to hear even a 2021 is having this problem. Wondering if/when GM started using the newer thermostat part number that has better specs.

 

I've got a 2020 SLE that's had the thermostat issue in the past, and it cleared once I cleared the P0128 code using an auto zone code tool. It was clear for about 8 months and it just recurred today- check engine light, steering assist reduced, warm AC, and no reading on temp Guage on dash. Had the code checked at a local dealership on way back to office and they confirmed it's a P0128 and they see it all the time on the ST6's ( I happened to be near a cadillac dealership).

 

I researched this as my 2017 acadia slt had same issue.....on another forum i found a few ppl that agreed this was due to bad coolant temp sensor that is less than 20 bucks bought aftermarket,but issue is the location of the sensor which i was told requires removal of valve pan and subsequently replacement of gasket for that pan. QUICK NOTE: removal of the neg batt terminal temporarily resets this issue as other senors also dont work when this error message displays.

 

Sry maniford gasket

 

I took my 2017 GMC Acadia SLT to my dealer yesterday (9/4/2024) for the "steering assist reduced" issue. I fortunately have the GM extended warranty, so the diagnostic testing was free. Testing diagnosed faulty thermostat. I left vehicle, they had the part in stock. Technician replaced thermostat, housing, and associated sensor(s) today (9/5/24). Oddly, they did not even require me to pay the $100 deductible. GM must recognize this is a widespread issue, probably should be a recall.

Anyway, so-far-so-good.

 

So did that help you issue?

 

I recently had the steering assist warning signal come on my 2018 Acadia Denali and at the same time. My AC quit working. I tried to have the system recharged by a local mechanic and his meter read that system was contaminated.
I drove to the dealer to have AC checked and on the way there a little red light by the gas gauge came on.
The dealer checked the AC and he said that the system was over charged so they had to drain it completely checked, for leaks, there were none, so the system was recharged and works well after $400
Now they want me to replace the thermostat and coolant temp sensor for $914.
I still have the original battery and they always give me thumbs up when I do oil changes.
After reading all of the comments maybe I should just get a new battery.
Any suggestions?
Bill 401996 3722
Thanks

 

Your battery needs to be replaced. When you get steering assist warning signal and the temp gauge not working and no AC, your battery does not have enough amps to allow your vehicle to operate properly. For now, just disconnect the negative battery cable and wait about a minute and reconnect. This will reset the computer and everything will work. This is only a temporary fix until you replace the battery.

 

Did your check engine light also come on and did your temperature gauge drop to 0? Certainly sounds like your thermostat failed and not your AC.

Sounds like you local mechanic failed to check the status of your AC charge before adding more. An overcharged AC is just as bad as an undercharged AC.

Sounds like the thermostat and sensor need to be replaced (a common failure and the price is about right).

 

I have a 2018 Acadia Denali & it is showing the Steering Assist Error Message which I believe is something with the Thermostat. Can someone please quickly explain the cause of this problem & what the Solution is? I want to be able to present it to the dealer if they try and tell me the problem I'm having is something else.


Thank you,

Dan

Change the thermostat. Problem solved. It sticks open causing all the issues.

 

The ambient temperature sensor will also cause the AC to not work. It's the sensor that displays the outside temp. It happened in my 2016. When it fails it will make the outside temp show way wrong numbers. If it is show correct temps then it's not that.
Good Luck

 

And. . . more importantly . . . replace the engine coolant thermostat with the newer upgraded one that has the proper intended operating temperature and doesn't stick open.
That is the real cause of this problem.

 

I have a 2021 acadia denali. I've had nothing but issues with this car. November 2024 Is when the steering assist reduced message came on. Subsequently the Fan goes into high power mode after I have turn the car off, And runs approximately five-ten min and then the fan turns off. We've already replaced the infotainment center, rear wheel bearing, sunroof track system, and can't use phone charger pad bc apparently it isn't compatible with my or my husband's phones. We have less than 50k miles on it and bought it used with 28k in 2022. It's now in the shop again for steering assist reduced message and a water leak inside the cab, drivers side by windshield. Have not replaced battery in car but might considering what's been said within this feed.

 

I had the same issue. The thermostat and a temperature sensor was replaced. During the inspection, dealer found that my radiator was cracked, had to replace that as well. Got a coolant flush and new coolant since I was doing all those things. I also have less than 50k miles on my 2020 acadia at4. Also replaced the battery because auto stop/start wasn't working? Apparently battery must have enough power to handle stop/start and as the battery gets old, this feature stops working. Too many issues with these GMCs and warning messages aren't clear on what's the failure.

 

The problem is that the thermostat is weak due to age and therefore allowing cooler water to enter engine than what the computer tells it should be. If you are going to replace just the thermostat, you are wasting your money. You have to remove the intake to get to the thermostat. Since you are already removing the intake, replace the sensor, spark plugs and coil packs. Some will disagree, but you will save money. A dealer quoted me $1200 to replace thermostat and I told him I wanted the rest replaced also and the price was $1700. However, at 74, I did all the work myself and parts cost just a little more than $300, all from amazon (2019 Acadia). Just look on YouTube and you will learn how to do it.

 

So now it is that it pays off to repair absolutely yourself and buy the tool because mechanics want too much for repairs. brakes took 860 for alternator wanted 770. etc. generally if acadia has over 100k you would have to throw it away or pay 10k extra for the whole restoration including engine and tramsis. these cars are some built to 100k and thrown away like disposable lighters. I repair my own acadia so I know how many problems such a gmc engineer caused for people but I think that now all cars are so badly tested. better buy horses / generally I suggest to get together locally and have a couple of people your own shape. order tools and one person is permanently responsible and lends these tools and takes care of the order etc. etc. etc.

 

Apologies for a novice question, but… What I don’t understand is, if the thermostat is not working, why does the engine temp gauge go from flatline to perfect temp as soon as you clear the check engine code?

i get that the thermostat needs to be replaced because it’s parameters are off and the symtoms kick in (fan goes into jet takeoff mode, steering assist turned off, coolant temp gauge flatlines to 0), but if the thermostat is not actually doing it’s job, why does everything snap back to normal, including the ACTUAL coolant temp when the code is cleared? If the temp gets out of whack, all the other things make sense as a self defense measure by the car’s computer, but clearly the actual coolant must be in proper range the whole time if within seconds the gauge instantly goes from zero during the “fault” to middle range (perfect) when cleared. I mean if the temperature is exactly what it should be, then how can the thermostat not be doing what it’s supposed to do? And if the temps are spot on, than what is it about this thermostat being inappropriate that makes the new one necessary?

 

That question has been answered numerous times in at least three threads.
The T-stat does not fail consistently. It can be OK one moment, then stay open too long the next and suddenly react to correct the out of range condition.

Suffice it to say. .. . It is the "temperature out of range" that is the cause for all the symptoms due to the T-stat either opening too soon or staying open too long which the computer sees as running cooler than expected. Out of range. The programming doesn't differentiate between too hot of too cool of the coolant temperature. Why? Too much to explain and it's in other threads. It is just what the programmers did to cover too hot or too cool.

All we really need to know is. . the newer part number T-stat runs in the expected range.

Here is a description from an earlier thread where the coolant was being constantly monitored by an ODB scanner and measured with the original T-stat that was failing on and off over weeks . . . and what was measured with the new GM part number - -

" - - - - the test drive with new T-stat the scanner was showing regulating at 201F on the highway. The original (which was not stuck open) was regulating at about 165 - 170F. "

That is it the fact.

 

OK. That makes sense. Thanks. I once drive 120 miles in 95º weather and the AC wouldn't work. Then all of a sudden, almost at my destination the temp gauge went back online and the AC came blasting out. When I finally discovered what was going on, the clearing it could last for hours or months. It hasn't been an issue this winter, but expect I'll eventually just pony up.

 

For sure. . . . once summer arrives and hot temps it is likely to start the issues again.

 

Agree with @divotdug. I've observed particularly starting about 2000, which already had vehicles start to load up with tech and difficult to get at assemblies, that vehicles across all brands have more potential areas subject to maintenance and earlier repairs.
I also do not recall owning or keeping a truck, car or SUV for more than 45K or 50K miles in the past 30 years or so.
There may have been an outlier or two if I added in kids cars as they entered mid high school and college.
I still do lend a hand at maintenance and light repair/replacements of parts now and then.
After which, muscles from arms, torso, back and legs will complain for more than a few days. I may take an ibuprofen or two and that will shorten the pain time.

At any rate, even things like T-stats, purge control valves and modules, PCV parts seem to experience early failure. Add to an engine a turbo, Gasoline Direct Injection and transmissions that are electronically controlled as well a Torque Converter Clutches and the areas of wear can multiply even under 50K or 60K miles depending on some factors and vehicle use.

At least, these are a few things I've noticed.

 

Did you get a solution?

 

i FIX MY SLLF

firsta time cost tools lader only parts

 

You also have to remember not only to wash the outside of the car but also inside the engine, transmission and power stirring. On YouTube I describe all how to do it. Engine every 3000 changes, add special fluid to flash engine. Transmission every 30 k, power stirring pump same time as transmission. and I started doing this when I disassembled and I see a lot of sludge or as if the oil was old.. You can see it when you disassemble to replace the timing chain etc. you should wash it now. this is what ruins your engine etc.

 

I had my 17' done under extended warranty. Issue has not returned thus far. Summer shall tell the truth. I had my plugs swapped for $50 (I bought the Delco factory plugs myself and they spent 15 minutes putting them in). Beware the dealer may be sloppy. I had residual coolant left in the valley that took awhile to burn off and they removed the sound deadening foam doing the work and didn't replace it. I called them on it and they fessed up, but some might now even know to look for it.