Several days ago, my CEL came on. I got the famous P0128 code, which as a lot of you already know, is thermostat, coolant temp. sensor, etc. related. Also, coolant level. OK. I checked that, and the level is perfect. I did notice a slight drop in coolant temp. on the gauge, from the normal "just over 80C" to a bit under 80C. It had never done that before, but I chalked it up to the fact that it's actually cold here in Arizona at night now. And that's about the time the gauge started reading a bit low for the first time. My dealership guy (friend is a service writer for them) told me it's common and the cause is USUALLY a leaky thermostat because there is an 0-ring under it that is famous for leaking and causing the thermostat to act like it is more open than it should be, thus causing the car to warm up more slowly, and THAT is what triggers the check engine light. OK. I don't do anything to fix it yet. Now fast forward to just the other day with everything the same. No changes made.
Now here's the unbelievable part: Ready? I go out to start the car day before yesterday, and it starts up and immediately dies. Turn the key, it starts, and shuts off. Great. Then I try to start it again, and it is dead. Like a completely dead battery.
I jump the car to get it going, and it starts right up. Ah! BATTERY problem! So, I get out my voltmeter and hook it to the battery and it reads 12.2 volts sitting there and I start the car up and bring the RPM's to about 1,500 or 2,000 to see what the alternator is putting into the battery, and it's about 14.2 volts. Perfect! Hmmmmmm. What the heck is going on here. So I take it to the shop and they said before they do anything, they have to hook it up to the diagnostic computer and see what's what. The guy says this is often a "key" problem. They come back and say it's telling them the battery is bad. OK. Fair enough. Even though it showed slightly over 12V, I understand there can be internal problems with one or more of the cells and that will do it. They put a new battery in it, clear all the codes, re-synch the windows, sunroof, and radio, and I'm good to go. I tell them I will deal with the thermostat problem myself. I leave the place and look at the coolant temp gauge and it now reads slightly above 80C! The problem is GONE! I've driven it now for a couple of days and the CEL hasn't come back on and it doesn't need a dern thing.....not a thermostat, not a sensor, nothing. On top of that, with the new battery (I know you won't believe this but it's true).....my gas mileage went up considerably! It was suffering a bit these past few weeks, but I thought it was the weather or something else. Nope. The dern battery. Also, the garage door opener on the mirror works at a greater range, and I'm convinced now the car actually runs better all around. I call my other automotive tech that I often speak with, and he says he has seen low batteries in these cars SEND A P0128 CODE!!!

So, I'm not here to say that all codes of this type are your battery, but it sure was in my case. Make SURE your battery is fresh and not on the verge of going bad. It can affect everything in these computer controlled cars! It runs better now than it has in months, and as some of you know, it has 148,000 miles on it now (2005 C230K). Yeah, I drive it.

Hope this helps somebody out in the future.

 

I forgot to mention that the new battery in my car has had another unbelieveable benefit.....my gas mileage has gone up considerably!
Now, I know what you're thinking....no way that's possible, right? Wrong. It went up a LOT. Especially on the highway I drive every day the same route and the same speed. I can sit and watch the onboard fuel mileage as I drive, and it's up quite a bit. Everything else the same. I wouldn't have believed it myself.

 

Interesting! I get the p0128 code somewhat regularly and it goes off and on in about 1 week intervals. I topped off the coolant just to be safe, but I monitor the temp gauge regularly and it is always in the middle and never gets too hot or doesnt warm up in a timely manner so not sure what is setting it off. I havnt experienced and other problems though and car runs good and fuel economy is still good so Im not sure if its the battery or not. Maybe its the thermostat/o-ring?

 

Created an account just to say this was my exact solution with the same symptoms. Weak garage door distance was what triggered me to replace battery. My other symptom was break hold not working didn't think anything of it and let go for over three months. Everything is working now with no issues! Tested old battery and it was weak also only pushing 200 CCA.
Thank you!

TEST YOUR BATTERIES!

 

thanks for the quide.

 

I Also Created an account just to thank you.
I Have a 2009 GL320 bluetec and I got the p0128 code. I literally replaced 4 thermostats. 3 were off Brand (I blamed the offbrand) and 1 OEM Mercedes. All showed code again within 3 starts.

I nearly bought a 5th one until I saw your post. I loaded tested the Battery. This is KEY if you want to know if battery is good. My old battery started car, showed over 12 V and everything worked BUT when loaded tested got into yellow gree zone.

A brand NEW battery was load tested an Always in the GREEN zone. I installed it and 70 starts and code never came back.

I just want to say Thank you again. It js hard to believe but it is the cause of the p0128 a weak battery.

 

I'm another one who made an account just to say thank you in advance of trying a new battery. Of course, I'll have the existing battery load tested first. It sounds like the leaky O-ring may just be a made up excuse by mechanics who don't really now the cause. It's human nature for professionals to want to have an answer.