W212 E350cdi Battery Indicator Illuminated
Posted: Tue Sep 22, 2026 1:40 pm
The battery indicator is illuminated.
Upon inspection of the codes, it indicates B21DF71, signifying a malfunction in the "Decoupling of starter battery" relay.The actuator is obstructed.
I have substituted the AUX battery, yet the same indicator illuminates. I have substituted the relay adjacent to the AUX battery in the rear, yet the same error persists.
I have detached the minor connector indicated in the photograph from the relay, and there are no errors displayed on the dashboard. I have operated the vehicle in that manner for approximately a week without encountering any issues, and subsequently, the AUX BATTERY warning illuminated on the dashboard. I have reconnected the connector, and the AUX BATTERY indicator has extinguished, yet the red battery light has illuminated once more. Could I be so unfortunate as to replace two relays, only to find that both are defective? There is an issue that triggers the red light when this is connected.
Upon inspection of the codes, it indicates B21DF71, signifying a malfunction in the "Decoupling of starter battery" relay.The actuator is obstructed.
I have substituted the AUX battery, yet the same indicator illuminates. I have substituted the relay adjacent to the AUX battery in the rear, yet the same error persists.
I have detached the minor connector indicated in the photograph from the relay, and there are no errors displayed on the dashboard. I have operated the vehicle in that manner for approximately a week without encountering any issues, and subsequently, the AUX BATTERY warning illuminated on the dashboard. I have reconnected the connector, and the AUX BATTERY indicator has extinguished, yet the red battery light has illuminated once more. Could I be so unfortunate as to replace two relays, only to find that both are defective? There is an issue that triggers the red light when this is connected.