When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I've got a 2017 C43 AMG with ~60,000 miles on it and a check engine light has come on twice now with a P0128 code. It first came on ~3 months ago, I went to a local auto-parts store to grab the code and then proceeded to take it into the dealer and get it checked out. They said I needed to get the thermostat replaced and quoted me $2100 for the work/parts.
Naturally I was a bit stunned at the price so I ended up picking up my own cheap OBD2 scanner and cleared the code - hoping that if it was a problem it'd pop back on. No issues for a month and it just came back on today with the same code. My thinking was - I've had no symptoms of a faulty thermostat, heat works fine (I live in the Midwest so its been frigid), engine temps seem normal never really going above 80 degrees Celsius, and no other obvious symptoms of a problem. When I explained this to the dealer the young tech's response was "the computer is saying that it may fail soon."
I'm not particularly mechanically savy but I have changed a thermostat in a car that provides much easier access. Given the complexity of replacing one in the C43 I don't feel comfortable doing it myself but I was wondering if anyone had any advice on other things to check to verify there's a problem or if there's a simpler and cheaper solution before I dive head first into a $2100 repair.
Any ideas? Is there a sensor I can access and replace that may be the culprit?
I am having a similar issue as the OP on my car - P0128, but functionally everything appears to be working as expected. We ran a few tests and the thermostat is not stuck open, the coolant circuits were gone through again to make sure they were properly bled and no air was in the system, and the coolant level was checked. The car drives fine, heats up fine, etc.
In addition, we logged the car to see when the thermostat is opening to see if it may be occurring early. While there is nothing conclusive yet, one thing I am trying to figure out is where the 2nd coolant temp sensor is. The data show 2 temps sensor readings which are different as the car heats up, but evens out once the thermostat opens. I started a thread asking if anyone knows the location of the 2nd sensor as I have not been able to locate it yet.
For those looking to change the thermostat themselves it is not difficult but can be a bit time consuming as you need to remove the intercooler, oil filter and a few other items to properly access the 3 screws that hold the thermostat on.
Note - A full service including a coolant flush was done towards the end of November and the code started once the frigid cold temperatures hit in my area about a month later.
I am having a similar issue as the OP on my car - P0128, but functionally everything appears to be working as expected. We ran a few tests and the thermostat is not stuck open, the coolant circuits were gone through again to make sure they were properly bled and no air was in the system, and the coolant level was checked. The car drives fine, heats up fine, etc.
In addition, we logged the car to see when the thermostat is opening to see if it may be occurring early. While there is nothing conclusive yet, one thing I am trying to figure out is where the 2nd coolant temp sensor is. The data show 2 temps sensor readings which are different as the car heats up, but evens out once the thermostat opens. I started a thread asking if anyone knows the location of the 2nd sensor as I have not been able to locate it yet.
For those looking to change the thermostat themselves it is not difficult but can be a bit time consuming as you need to remove the intercooler, oil filter and a few other items to properly access the 3 screws that hold the thermostat on.
Note - A full service including a coolant flush was done towards the end of November and the code started once the frigid cold temperatures hit in my area about a month later.
A second sensor is mounted on the radiator itself.
What do you think triggers the fault if TSTAT works perfectly normal??
Last edited by CaliBenzDriver; Jan 27, 2025 at 06:56 PM.
@CaliBenzDriver thanks - I saw your reply on the other thread and appreciate the info!
To note, I am not saying the thermostat is working perfectly, but it does not appear to be stuck open. It is a small possibility that it could be the coolant temp sensor - I doubt it but want to rule it out. I am leaning towards the thermostat opening early, but I have not been able to confirm as I have been too busy tuning a customer's car to focus on mine.
@CaliBenzDriver thanks - I saw your reply on the other thread and appreciate the info!
To note, I am not saying the thermostat is working perfectly, but it does not appear to be stuck open. It is a small possibility that it could be the coolant temp sensor - I doubt it but want to rule it out. I am leaning towards the thermostat opening early, but I have not been able to confirm as I have been too busy tuning a customer's car to focus on mine.
What MB has developed is a "gently failing Tstat".
They use the combination of mapped response (not direct) plus the Tstat has a "built-in heater" controlled by ECU.
I think ECU fault is to evidence Tstat getting lazy/hard to travel open/closed. It needs replacement for good measure before engine experiences more extreme heat.
+++ Smart TSTAT HEATER Ctl...
The feature is supposed to help ECU manage exact coolant temperature based on load, Rpm, speed, ... where the reality is slightly different:
to help diagnose failing Tstat as proven bad.
Last edited by CaliBenzDriver; Jan 27, 2025 at 09:38 PM.
Quick update - the GR Corolla is gone and my schedule has allowed me to spend time on my car. As mentioned in a previous post my thermostat was not stuck open, but I was able to confirm that it is opening early. Initial data shows that the thermostat was opening at approximately 180 ° F. I believe that it's supposed to open at approximately 210 ° (please correct me if I am wrong). What is even more interesting is that opening temp has dropped over time - the last time I checked it was opening just shy of 170 °. I plan to install the new tstat soon (within the next 2 weeks 🤞🏿 and will report back once it is done.
Quick update - the GR Corolla is gone and my schedule has allowed me to spend time on my car.
As mentioned in a previous post my thermostat was not stuck open, but I was able to confirm that it is opening early. Initial data shows that the thermostat was opening at approximately 180 ° F.
I believe that it's supposed to open at approximately 210 ° (please correct me if I am wrong).
What is even more interesting is that opening temp has dropped over time - the last time I checked it was opening just shy of 170 °.
I plan to install the new tstat soon (within the next 2 weeks) and will report back once it is done.
A new trusted quality Tstat on schedule is a great idea because we never want to fiddle with poor cooling.
It may well be that managed Tstat opening is entirely based on learned data with very little to do about real-time sensor.
This would explain why ECU does open up Tstat way too much in winter at Hwy speed when it expects dry-pistons heat to get circulated out by more effective spraying RPM.
Meaning a low Hwy temp would be evidence of a learned response expecting extreme heat that's no longer stored by dry pistons.
At any rate cooler response is safer that extremely high!!
This mapped Tstat response does get relearned to become normally smooth.
Last edited by CaliBenzDriver; Feb 8, 2025 at 03:08 PM.
Hi all - quick update. I changed the thermostat a while back, and the P0128 code has not returned. I took the old thermostat apart, and while it may not be apparent in the picture below but the charge cylinder separated from the seal. Seeing this, the behavior now makes sense to me since the tstat was partially open but not enough to prevent the interior from heating up, etc.
Space between cylinder and seal
Unfortunately, now I am dealing with a misfire on cylinder 6; it looks like I have a bad injector. smh...
Mercedes SLR McLaren 722 S Is Extremely Rare Example Modified by McLaren
Slideshow: A one-of-one U.S.-spec Mercedes-Benz SLR McLaren Roadster became even rarer after a factory-backed transformation at McLaren's headquarters.
and will report back once it is done.
