In this guide, you will find instructions on how to replace the auxiliary battery relay (K57/2) on Mercedes-Benz vehicles. 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 "Auxilary battery malfunction" there is a chance that the auxiliary relay may be the culprit.
This guide only shows how to replace the auxilary battery relay. To learn how to replace the auxiliary battery itself, follow the guide on How to change Mercedes-Benz auxiliary battery.