Guys I have been getting the p0128 fault code. I have checked the antifreeze. I know this usually means the thermostat is faulty but the car isnt running hot. What do you guys recommend? Should I start by changing the water temperature sensor or only change the thermostat?

 

Here's a thread about it, maybe this will help some.

https://mbworld.org/forums/c-class-w...need-help.html

 

Also the car only has 36k miles. The car warms up just fine and the temp is 87'f. So I'm assuming the thermostat is stuck open.

 

I got the code on mine, I ordered a new thermostat yesterday. I am pretty sure mine is stuck open because the car seemed to take forever to get warm also MPG are lower than usual.

 

I replaced my thermostat yesterday. It was kind of a pain because you need to remove the belt, one of the rollers, and then also you need to loosen the air pump. I was very paranoid about stripping out the aluminum holes.
Also I think some of the rollers or the tensioner is on its way out.

After replacing I had to clear out the code, it wouldn't go away on it's own.

The old one was stuck open and the car wouldn't ever fully warm up.

 

I'm just now seeing this thread, but the lesson to be learned here is that typically automotive thermostats fail in the "open" position. They are designed this way so that when they fail, they do not destroy expensive engines.


We should be thankful that some engineer, long ago, designed these things sensibly. . .

 

Well,
I have had only two thermostats fail on my past cars and both of them failed closed.
The thermostat closes with spring and opens by the heat expansion of the wax that presses the "valve" open against the spring. The steel spring is not likely to fail but the heat expansion wax material in its compartment is meaning when the thermostat fails it likely fails closed. At least mine did.

 

another good thing about the thermostat design is that it has a small hole in it so that in a worst case situation at least a little pressure can escape.