Measure voltage at BMS master connector – must be >11.5V during cranking.
This is a detailed technical paper on the Mercedes-Benz fault code/trouble code , based on Mercedes-Benz WIS (Workshop Information System) documentation, XENTRY/DAS diagnostic data, and common failure patterns observed in the field. Technical Service Paper: Diagnostic Trouble Code P1D687A Manufacturer: Mercedes-Benz DTC: P1D687A System: High-Voltage (HV) Battery Management / Battery Energy Control Variant Relevance: Primarily EQ Boost (MHEV - Mild Hybrid Electric Vehicle) with 48V onboard electrical system (e.g., M256 engine in W222 S-Class, W223, W205, W213, W167 GLE) and full BEV/HV systems (e.g., EQC, EQE, EQS). Severity Level: 2 (Reduced performance, warning lamp, possible drivetrain derate) 1. Code Definition P1D687A – Communication with the high-voltage battery management control unit has a malfunction. The signal path is faulty. mercedes-benz p1d687a
Perform HV isolation test to rule out moisture ingress into battery pack (which can short bus lines). Measure voltage at BMS master connector – must be >11