In this guide, you will find instructions on how to replace the auxiliary battery relay (K57/2) on Mercedes-Benz vehicles.

The Mercedes auxiliary battery relay is typically mounted at the top of the firewall, right below the windshield wiper motor.

If you have replaced the auxiliary battery and are still getting the message, "Auxilary battery malfunction" there is a chance that the auxiliary relay may be the culprit.

What you will need


  1. Park your Mercedes-Benz and set the parking brakes. Remove the key from the ignition. parked mercedes
  2. Open the hood by pulling the release under the dashboard.
  3. Next, you need to remove the wiper arms. Remove the 13mm bolt on the wiper arm. Pull up the arm to remove it from the shaft. remove wiper arm
  4. Remove the plastic trim. There are four plastic retainers that will need to be removed before you can remove the plastic trim. remove plastic trim
  5. Locate the auxiliary battery relay mounted on the firewall. Remove the two bolts that hold the auxiliary battery in place. Unplug the electrical connector and replace it. Mercedes auxiliary battery relay location

This guide only shows how to replace the auxiliary battery relay. To learn how to replace the auxiliary battery itself, follow the guide on How to change Mercedes-Benz auxiliary battery.