Auxiliary battery malfunction warning: causes and fix

Which Mercedes models are affected?

The “Auxiliary battery malfunction” message appears on a range of Mercedes-Benz models. For other Mercedes work, see our Mercedes-Benz specialist page.

• The E-Class (213) owner’s manual lists the message with the transmission messages.
• Mercedes workshop information on the fault covers the A-Class (176), B-Class (246), CLA (117), GLA (156) and M-Class (166).
• According to one UK parts retailer, the C-Class (205), E-Class (213), GLC and S-Class (222) use a capacitor-type voltage converter in place of a small battery.

Where the auxiliary battery or converter sits depends on the model. Put your registration in the enquiry form and we will check what your car has and where it is.

What are the symptoms?

Often the message is the only sign. Owners and parts suppliers also report that the start-stop system stops working.

On the E-Class (213), the auxiliary battery backs up the gearbox park function. Until the fault is repaired, Mercedes says to select P manually and apply the parking brake. Check your own handbook for what it says about your model.

When you book, tell us:

• when the message appears, for example at start-up
• whether start-stop still works
• whether any other warnings are showing

What causes it?

The message does not always mean the auxiliary battery itself has failed. The causes named in Mercedes workshop information and by specialists include:

• An aged auxiliary battery or converter. As it ages, its internal resistance rises too high for it to do its job.
• A weak main battery, which can produce the same message.
• Resistance in the wiring between the auxiliary battery and the rest of the car.
• A control unit fault, in the signal acquisition and actuation module (SAM).

How do you confirm the fault before fitting parts?

A warning message is not a diagnosis. Before we quote for any parts, we:

• test the main battery first, because a weak main battery can trigger the same message
• charge the auxiliary battery, then measure its internal resistance repeatedly with our diagnostic equipment, following Mercedes’ test procedure
• check the resistance of the wiring, to tell a battery fault from a wiring fault
• read the fault data stored in the car’s control units

We then show you what we found and send a written price. No part is fitted until you give us your written approval.

What does the repair involve?

We replace the part the tests prove faulty. Depending on the result, that is the auxiliary battery or converter, the wiring, or the main battery.

Once the work is done, we clear the stored fault, check that the message has gone and that start-stop works again, and verify the repair.

How long does an auxiliary battery repair take?

It depends on what the tests find and on where the part sits on your model.

Before any work starts, you get a written price and a promised time for your next update. If the car needs to stay with us, ask about a courtesy car when you book.

Is there a recall for the auxiliary battery?

DVSA does list two recalls for voltage converter software on the S-Class (223), R/2023/148 and R/2023/149, but they concern a different fault.

Whether a recall applies depends on the individual car, so check your registration on gov.uk. Recall work is done by the manufacturer’s network, not by us.