FIXED - ECO start/stop not working on Mercedes E350 W212; was NOT the auxiliary battery in the trunk

The Start-Stop ECO feature on our E350 (W212) stopped working. It was not a weak auxiliary battery (2115400001 or 0009829608) in the trunk. The problem turned out to be solved with a new main battery (0009822108). The main battery initially showed a low voltage reading of 12.1V and my external charger showed the battery at 55% charge. I charged battery to 100% and then took it to be load tested at local auto parts store. They confirmed battery was weak and delivering less than 50% of rated Cold Cranking Amps. ECO problem resolved with new main battery. I also upgraded the battery from original H7 to a larger and more powerful H8 which easily and properly fit into the existing battery tray at the same price as the H7..

[@KarimKhan-gy3ks]

Thanks, I'm going to try this. My auxiliary battery had fine voltage, though I did once get an auxillary battery malfunction warning (only once for some reason), and I changed it out. Still no ECO mode, it doesn't even show up in yellow on the intrument panel. No signs of a weak main battery but seeing your video, I'll get it checked.

[@jerrihalf5564]

I was researching why my ECO mode no longer kicked in on my 2015 and came across your vid. I also noted no start-up drag or anything else that would have led me believe my battery/ies were tired. But I then realized I still had the OEM and they were 7 years old so I replaced both. The dreaded ECO mode is back, but now I don't have to worry about getting stranded somewhere.

[@victoripeters1]

Thank you so much. I also changed the auxiliary battery, eco mode still didn’t work then I just bought the battery under the hood and it’s finally working. Thanks for your video

[@YuDaMan]

Good job. You did it yourself and saved $$$. Be sure to LIKE and Subscribe. Your AUX probably wasn't far behind the main in terms of life. Be sure to LIKE and subscribe. THAT would really help me. THANKS

[@keithlau199]

You mean you changed the main battery ?

[@DunsKicks]

Appreciate that boss👊🏾

[@jamesbambury]

If this feature stops working I would consider it to be the car self improving.

[@MasterBait1]

But you still need to manually turn it off, mine shuts the engine for me, but never put it back on.

[@mistadontplay1231]

Hmm I wonder if this is the same thing for my car….I do know I had to jump my car when it was really cold but I haven’t noticed any starting issues or drag with the started I’ll check into this to see

[@tpdtopcop2010]

Where’d you get the battery from? We don't have a pep boys where I live. I checked my main battery and it's at 12.1 volts. Might also be my problem. It is the original MB battery in my 2014.

[@YuDaMan]

I'm sure your local autozone, advance auto or maybe even WalMart carry the battery you need, either H7 or H8. My original was an H7 but since an H8 would fit my try, I used an H8.

[@tpdtopcop2010]

@@YuDaMan Thanks. Going to use H8 as it has more CCA.

[@tylertran93nsx]

So I have a 2012 Mercedes E350 W212 and the ÁC button wont turn off, it’s the main battery?

[@YuDaMan]

sorry I do not know the answer to that question

[@yawor1]

I replaced both my batteries, auxiliary and main batteries on my w212 e class with original Mercedes battery but my start and stop still not working. After driving it for 2 min instead the eco going green it actually just goes off, doesn’t even stay amber. Any one know what it can be?

[@MP--zs2iv]

Did it eventually get fixed? I also replaced both batteries. Mine doesn’t even light up

[@yawor1]

@@MP--zs2iv unfortunately not… cant be bothered no more. Use to it not working, and dont want to throw anyone money on it.

[@djn1856]

Did u have the “auxiliary battery malfunction “ warning light?

[@YuDaMan]

no warning lights ... no indicators ECO not working other than ECO was not working Dealer said most likely the aux battery and not main battery, based on their experiences. It was NOT the aux battery. New main battery solved the ECO issue

[@djn1856]

@@YuDaMan thank u for the detail reply. I am in similar position, eco not working but i had “auxiliary battery malfunction” warning light… i changed the aux battery and the warning has gone, but still no eco lol. I will try the main battery i think

[@YuDaMan]

@@djn1856 good luck. be sure to report back since that will certainly help someone else. thanks!

[@flintsgaming428]

@@djn1856 I had this problem changed both batteries still getting the same problem Maybe the relay

[@wutangpaul]

So did changing the battery solve the problem? You didn't say in the video

[@YuDaMan]

Yes, changing the MAIN battery solved the problem. Dealer said in advance it would be the auxiliary battery which would have been cheaper and easier but my aux battery tested good.

[@wutangpaul]

@@YuDaMan Awesome, thank you. Having the same issue with my E350.

[@flintsgaming428]

I’ve got this problem I’ve changed both battery’s but still not working I’m thinking it maybe the relay not working ?

[@verticalpicsllc5575]

Did you fix it? Was it the relay? If so post a link to the part please!

[@deriklongokailer]

Hi i have this problem, i changed both batteries, the voltage sensor on the main battery, and the relay, still not working. Please let me know if anything worked for you, because i am starting to suspect someone sold me one of these items faulty

[@georgeanddaddecker7563]

You got nine years out of your original battery?

[@TommyMissus]

U mean ur eco doesn't work? You're so lucky. I wish mine didn't work

[@coloradonativefishingcnf6175]

My engine light came on in my car started driving different. Did your wife still the same?

[@MasterBait1]

Either mine in the boot was dead dead, or my used car dealer bamboozled me with a main battery that is not new

[@manofwar577]

Wish mine would stop working!, the most damage done to engines is dome durig start up!, common knowledge!

[@YuDaMan]

just turn your ECO OFF using the button!

[@mikea6773]

I wish mine would quit working if I could break it I would

[@astrasfo]

Watch youtube video on how to disable eco start.

[@samanazad6704]

You was excited about price 🤪instead of main problem 🙈🙈🙈 Only think was clear explain 👎👎👎

[@mariosturtz4835]

As if anybody watches this video to find out what you’ve paid for your battery. Ridiculous video