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ECO Start/Stop Inactive Message & Auxiliary Battery Question on 2018 S560

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Sarah K. · rep 0 · May 22, 4:22 AM · 1 replies · solved · Question

Vehicle: 2018 Mercedes-Benz S560

For the past few weeks, my 2018 Mercedes-Benz S560 (W222) has been consistently displaying the 'ECO Start/Stop Inactive' message. The ECO button light is off, and the engine never shuts off at stoplights. I've also noticed that sometimes the car takes a bit longer to crank, especially on colder mornings.

I know the ECO Start/Stop system relies on an auxiliary battery, and I'm wondering if that's the culprit. The car has about 55,000 miles on it, and I haven't replaced any batteries yet. Is this a common issue for the S-Class, and how can I confirm if it's the auxiliary battery or the main battery?

What's involved in replacing the auxiliary battery, and is it something I can do myself, or should I take it to a shop? Thanks for any insights!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 4:22 AM

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ClearTheCode admin answer:

The 'ECO Start/Stop Inactive' message on your 2018 S560 is a very common indicator of a weak or failing auxiliary battery, or sometimes the main starter battery. The ECO Start/Stop system requires optimal battery voltage and health to function correctly, as it places a significant load on the electrical system.

Common Causes for 'ECO Start/Stop Inactive':

  1. Failing Auxiliary Battery: This is the most frequent cause. The auxiliary battery (often a smaller AGM battery) powers accessories and supports the ECO Start/Stop function. As it ages, its capacity diminishes, and the system disables itself to prevent issues. You might see codes like B182F00 (Auxiliary battery has a malfunction) or B220400 (Auxiliary battery voltage too low).
  2. Weak Main Starter Battery: If the main battery is also weak or nearing the end of its life, the vehicle's electrical system may not have enough reserve power to reliably restart the engine, causing the ECO system to deactivate. The longer cranking time you mentioned could point to this.
  3. Other System Conditions: The ECO Start/Stop system is designed to be inactive under various conditions (e.g., engine not at operating temperature, climate control demanding high power, low outside temperature, steering wheel turned sharply, DPF regeneration). However, if it's always inactive, battery health is the primary suspect.

When to See a Shop (or DIY):

While replacing the auxiliary battery can be a DIY task for some, diagnosing the exact cause and ensuring proper system integration is best left to a professional. Here's what to do:

  1. Battery Test: A Mercedes-Benz specialist can perform a comprehensive test on both your main and auxiliary batteries using specialized equipment to determine their state of charge and overall health. This will confirm which battery (or both) needs replacement.
  2. Diagnostic Scan: They will also scan for any stored fault codes related to battery management, power supply, or the ECO Start/Stop system.
  3. Auxiliary Battery Location: In many S-Class models, the auxiliary battery is located in the trunk, often behind the right-side trim panel. It's typically a smaller 12V battery. While physically accessible, proper disconnection and reconnection procedures are important to avoid electrical issues.
  4. Main Battery Location: The main battery is usually under the hood or in the trunk, depending on the specific S-Class variant.

Replacing the auxiliary battery is generally straightforward, but if the main battery is also weak, or if you're unsure about the diagnosis, a professional will ensure the correct battery type is installed and any fault codes are cleared. Expect the auxiliary battery to cost a few hundred dollars plus labor if done at a shop.

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