JLo99999

I have had the aux battery failure, and replaced it myself, however I still have the white failure message coming up. I have in the past had the red message too. All stop start functions are working, I just have the annoying message which keeps popping up every 5 or 10 seconds. Does the error code need to be cleared using Xentry, by MB?

C300Paul

It’s just the standard battery that needs changing. It’s actually a capacitor rather than a battery. It’s not the hybrid batteries.
the W205 has an issue with them. They had a recall a couple of years ago so you should check and maybe get it for free. Batteries aren’t normal part of warranty as they’re classed as wear and tear.
they only seem to last about 2 years.
you can still drive the car no problems.
the stop start might not work otherwise no dramas.
best to get it changed at some point as some sensors on the car need stable power to operate properly

eddieo45

All good info there. My C-Class had over 60,000 mile, over 4 years old when dealer replaced it free. My SL-Class was still under warranty and its auxiliary battery, which is an actual battery, was NOT covered.

mbboye

@ times its depending on what voltage the Aux is converting, they have separate modules that control separate voltages. 4 EX. fuel pump module controls the fuel pump voltage.

Greg52

The confusion comes from the message displayed when the device fails, the display says auxiliary battery malfunction when in fact it is not a battery but a capacitor bank and some other associated electronics. My understanding is that it is meant to maintain battery voltage during engine start up but no doubt has some other functions that are not obvious to the driver, the symptom that is noticed by the driver (apart from the error message) is that the start stop function stops working. It’s also known as a voltage converter, as well as a parking pawl capacitor. There have been several part number changes over the years as this troublesome component has been improved. There are Mercedes models that do have an actual auxiliary battery but not the w205 c class.

mbboye

I have a 2017 w205 c class the Aux Battery located under passenger foot rest

Greg52

It is in fact not a battery but a voltage converter/parking pawl capacitor, please read the threads

mbboye

If you go to mercedes benz they will tell you that is a battery/voltage converter its confusing to why people thinking a module cannot have over 1 function. Battery can either store power or convert power. hopefully you've undergone proper terminology schooling

Greg52

It contains no battery only electrolytic capacitors and some associated electronics, nowhere have I seen it described as a battery in any Mercedes publication or spare parts manual, happy to be proved wrong

MBenzOwner

Can this item fail and intermittently affect starting / restarting without an auxiliary battery malfunction message? Has anyone replaced this more than once?

Greeneyety

The exact same thing is happening to me now...suddenly the stop/start stopped working. No malfunction message or code and nothing else seems to be affected. I assumed it was the aux. battery.

PaulR7

If your start/stop system stops working with no aux malfunction message then the most likely scenario is you need to either charge or replace your main battery.
It's however possible the car doesn't meet one of the many other requirements for start/stop to actually engage like coolant or ambient temperature or you use kickdown too often etc.

MBenzOwner

Is your issue that the vehicle doesn't stop during the ECO start/stop process? As mentioned that is likely due to some aspect of the requirement checklist that is not met. I attached the guide that explains it. This is the intended function of the system to prevent ECO stop unless all requirements are met. If you can get access to a Xentry system, it should tell you what requirement of the process is not met. You would need to address that.

The battery voltage is a common reason. You can also access that through the secret diag menu which lists the voltage without xentry. There are youtube videos showing how to get that menu.

My issue is failure to restart after an ECO stop which is a system failure really because if some problem was known to the vehicle, it would never shut down in the ECO start/stop process. The only code I had was P1CE200. It went through the diag steps and even took it to the dealer without resolution.

Phil Jackson

• I have the same problem, have you managed to get the fault sorted?

denis550

I just changed the aux battery on my w205 MY15. After changing the module I immediately cleared all fault codes on my car. I had the fault B21DC01 "The buffer battery of the electronic ignition lock has a malfunction. There is a general electrical fault.". I don't know if the car would have cleared the code itself after driving it or not.
If you don't have a obd scanner you could first try checking the aux battery fuse. It's in the trunk fuse box, the fuse number is #448 Park pawl capacitor in the diagram, it should be a 10 amp fuse.

I hope this helps you.

Joseph Lavinski

I have a 2015 CLA (42,000 miles) with this problem and it is now at MB being serviced for this problem. There was no malfunction warning but the car would stall after Eco start/stop. Both the auxiliary and the regulator(?) need to be replaced. The car would work OK if I shut off the Eco start/stop/

Phil Jackson

Thank you, it was the fuse at fault

Joseph Lavinski

I just had my aux battery & relay(?) replaced on my 2015 MB CLA Cost was about $1,350. It is working fine now.

Snoop74

Hi All

Has anyone experienced the below reliability and build quality issues on 2015 C220 Diesel W205 (AMG Line) at 60k:

I bought the car at 50k miles with a FMBSH:

1. Nox sensor went at 52k - very expensive repair!
2. Front control arms gone at 55k - very expensive repair!
3. Auxiliary battery shorted (55k) and led to burning smell in cabin - very expensive repair!
4. Dashboard and door panels rattle and creak all the time - how on earth did MB put poor quality materials in such a nice interior!
5. 59k - Added Redex fuel treatment and engine light came on after 20mile run and no issue in power or drive- anyone know how to safely reset the engine light without going to MB garage for diagnosis and fix?

I cannot afford to sell the car so would rather keep it and put little maintenance work in it since I only do <5k miles a year.

Mike Salkeld

wow! This sounds so much like my c250d AMG LINE S205…

nox sensors failed at 45k miles (replaced then failed again)
lower control arms ‘failing’ at 47k miles
eco stop start not working due to battery drain

what did you decide to do?