I started getting the P0128 CEL about a year ago and had it reset about 6-8 times and did not worry too much because I always ran too cool so I figured the thermostat was stuck in a partial open position. But one of these days it might fail and that would have been bad!
Finally got the best of me and after much searching found the correct Wahler thermostat. You can see by the picture that my old thermostat was indeed stuck. About $120 but a lot of sources were out of inventory. Amazon, go figure.
Here's the deal. There a couple of videos on YouTube about doing this on the V-6 but not the V-8. Turns out to be very little difference
Run it up on the ramps.
Drop the front plastic pan.
Radiator drain plug, on my car, is on the side of the radiator with the valve right above it. About 1/4-5/16" tube will fit over the nipple. Don't forget to remove the fill cap on the reservoir. About a gallon will drain out. Mine was blue.
Remove the front engine cover and you will see the thermostat right in the middle of the engine. You will need some E sockets. Bought some at Sears for $12, 1/4" and 3/8" drive. The next thing you will notice is that the serpentine belt is in the way!
I found it easier to go under the car with a long handled 3/8" breaker bar attached to a 17mm socket to unload the idler pulley. Quite a bit of rotation.
The OEM belt was 2392 mm. Be careful to get the correct length, check you manufacture date. I paid $53 at the local dealer but could have gotten a Gates for about half but the replacement belt was 2390 mm? Conti. Anyway, you will need help putting it back on.
The next thing in your way is the idler pulley. No big deal, a T-30 or 35, and off it comes. It only goes back on one way.
The air pump sits above the thermostat and it interferes with the top thermostat bolt! It has three torque type screws that secure it. But rather than remove it, I loosened up the top one and the next to lower one and the top two hoses and it moved enough to get access to the bolt.
Now you can get to the thermostat housing! There is a sensor built into the housing at about the 4 o'clock position. One video recommended removing it before removing the housing. It's a pain and there is enough length in the wiring harness to remove the clip after unbolting the housing.
The two bolts are long and fine threaded. Takes a while.
My thermostat only required a light hit with a rubber mallet to come off. Clean the engine side, carefully, its aluminum!
The gasket that comes with the thermostat has small ears to center it through the bolt holes to allow the gasket to stay centered while you tighten it down. Thank goodness. Did not use any ATV since the gasket material seemed to be treated on both sides.
Do not over torque. Everything is aluminum!!
I reattached the temp sensor to the housing before reattaching. Clicks right in and the wiring harness falls into place.
Reverse the process. Air pump tightened and hoses reattached. Idler wheel reattached. Now the serpentine belt.
Get a good picture of how it is routed. It is a long belt! If you do not have three hands you will require help to get the tensioner out of the way and make sure the belt is lined up. I guess there is a slot on the tensioner that you can insert a small punch or pin into that will hold it but I did not see it and it was quicker just to have someone slip it over while I held it up while under the car.
Make sure the radiator drain valve is seated before adding coolant Mine was a little tricky but you will feel it seat. Mine had a white line added to it? Reference?
Used the concentrate coolant from the dealer. Blue and it can be diluted to 50/50 but use distilled water. Aluminum remember. About $24 and $1.50 for the distilled water. For insurance I bought a gallon of the Zerex G-05.
Took about 1 1/2 gallons, total, to fill. Interesting, only drained a gallon.
Fired it up. No leaks. Replaced the front engine cover. Off the ramps and let it run till it hit 87 degrees C. Got a low coolant light, topped it off and test drove it.
Took about 4 days for the CEL to clear. Could have been more aggressive and had it reset but I was curious
Hits 86-87 degrees promptly and everybody seems to be happy.
Hope this helps someone. No promises, no guarantees. Worked for me and saved about $500.

 

Good write up.
I just did this a few weeks ago. I guess the thermostats are junk.

 

You will need to remove the upper radiator hose the Mercedes wire bail clip is the slickest hose connection I've seen😄
Mine came off very easily and was a snap to reattach.

 

Great info! I'm going to be doing this this week along with my 40k tranny service, possibly the brakes, the serpentine belt+tensioner, and suppose to do brake fluid I believe.... Ugh... At least my new wheels are going on! Week after will be the summer 3 stage cut/polish/wax process.....

 

I'm at 55,000 miles and just had mine replaced yesterday with the same CEL code. $400 at my Indy. Not a job I felt comfortable doing, but props to all you DIYers.

 

Very nice writeup. I have never heard of 'E sockets'. Please elaborate.

 

I feel really good about this repair. Fortunately I have access to a full import auto repair shop and several mechanics I can ask a question to if I ever have a concern. I'm going to take on the tranny flush/filter at the same time which I'm a touch less enthusiastic about even though I have STAR for the refill. I know it's unlikely the car will be 100% level on the lift so reading up to ensure I do not have any issues on the back end.