Hello, Yesterday my Aux Battery(the trunk one) malfunction error showed up, then Distronic Plus was inoperative along with Hold and Eco mode features.
However after few miles, the main battery took over all these features, causing one issue:
Eco mode is on main battery and it works only one time per engine recycle: meaning I start the engine, charges for some time and ECO mode gets activated, and when I come for a complete stop and the engines cuts off, this time, when it starts the stereo also restarts and the ECO functions turns off completely until I restart the ignition.

I took the car to Advance auto parts and Napa to get 2nd opinion.
AAP said the battery is dead;
After driving for about 50miles, Napa said the battery test can't be performed and it showed 13.4V.

On the other hand I had my oil change done at 61k, ATF at 47k and the spark plugs needs to be changed at 60k and I'm at 63k. How long can I go until I replace them?

I purchased the Bosch ones on eBay for 45$ (after 10$ coupon). (ZR6Sii3320 - 7430)
MB Dealer asked: $518 + taxes when I bring my own plugs.

Indy Shop: 463$ with plugs
355$ BYO Plugs.

Tafel Motors(Jeff) quoted $1570 + taxes for 3-pack. Getting more quotes (Advice please).

1. Please share Battery code and which one do you guys suggest: AGM vs others? Size? (Can I do it on my own at Auto Parts Store to borrow the tools)
2. How many more miles can I go before changing the plugs? I want to request for less price at the indy shop as it shouldn't take more than 2hours in labor..!

TIA, Please advice.

Wait, did you replace the AUX battery or no? It should have nothing to do with the main battery, and you normally lose Hold and Eco stop/start when there's an aux battery malfunction as it's part of the system and requires no errors to be activated.

Aux battery malfunction can cause eco start/stop to not function properly. I've had this issue before and dealer swapped it out under warranty.

Spark plugs honestly I'd say dont worry about it until one of them actually causes a misfire. Then when that happens, its best to just change all of them out instead of having them fail one by one.

main battery is fine, but the features are relying on the main battery I believe because the ECO start stop works only once per Engine cycle and it restarts the stereo also..!

thanks for the adivce on spark plugs.
When you say warranty, 4yr/50k miles or ELW also to replace the Aux battery?

That's not how the system works. Replace your aux battery, but you may have a weak main battery as well or some other issue.

On my 211 my first spark plug didn't even go out until 90k miles.

My aux battery was replaced 1 month before my factory warranty expired. Cannot confirm if they are covered under elw, would suggest checking with dealer for that.

If your shopping for PPM, why not wait on the plugs until they cover it. ATF and plugs are how you get your moneys worth out of the PPM.

went to NTB to get the aux battery tested. They failed to test and suggest me the stealership.
Stealer asked me 130$ + taxes to diagnose alone..! I didn't dare to ask for the cost of the replacement.

Everything works fine and no 'aux batt malfunction' error on the dash, only problem is when ECO start/stop works it restarts the main stereo and ECO mode will turn off completely, until I restart the car. Will it harm the vehicle?

Vehicle is at 63k (oil change done at 62k) and ATF at 47k; I bought my plugs so, I'll wait until one of them fails as others suggested and replace'em all. Only thing to worry about is the brake fluid change as per manual them.
Probably I need to buy them after my 70k maintenance.

As mentioned, it sounds like aux battery is weak. Do a search on aux battery and you will find inexpensive sources for battery as well as how-to steps. I remember seeing these type of posts.

I've not found AutoZone/pepboys/ntb/etc to be worth a flip at diagnosing anything other than very standard generic stuff. Buy yourself a decent scanner and you can do better at checking your car.

I don't know how much DIY you do but brake fluid flush is pretty straightforward. Some use a pressure type bleeder, I use a Mityvac to draw fluid from the bleeders and replenish at the master cylinder. Total about 30-40 min and less than liter of brake fluid. I used factory type Pentosin fluid.

Yes, thanks. found the steps on how to replace it. Battery cost itself is 110$ and will buy that and replace if needed.

I did worked on my 2008 Yamaha FZ6 for oil changes and coolant changes, but I'm scared enough to touch brakes Definitely will not try on this elephant.

Here are a couple threads on DIY for the trunk aux battery (eco stop/start) version. These folks said it was about $140. I would just replace it rather than rely on main battery.
https://mbworld.org/forums/e-class-w212/620950-2014-e350-auxiliary-battery-malfunction.html
https://mbworld.org/forums/e-class-w...lfunction.html
Ignore the threads about the small aux battery in the dash that's only $15 on amazon. That's for 2012 and earlier.

Can you return the plugs? If you get PPM and it would cover the plugs, I am not sure why you would wait. I got my plugs done at 60k and ATF done at 80k with PPM.