C1A6F

Code

Generic C — Chassis

Intermittent Loss of Communication with Battery Management Module (BMS)

Brand: Generic

Type: C — Chassis

Views: UK: 14 EN: 22 RU: 12

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Page language: EN

Causes

Intermittent power or ground to the BMS (loose connector, corroded contact, blown fuse, faulty relay)
Damaged, chafed, or intermittent CAN/LIN wiring or splices between BMS and gateway/ECU
Faulty or partially-corroded BMS connector or pin
CAN bus termination problem or high bus impedance (missing/failed resistor or water ingress)
Module going to sleep or not waking properly (software/configuration issue)
Faulty BMS (internal failure) or intermittent internal electronics

Symptoms

Intermittent battery-pack state-of-charge (SOC) reporting or incorrect SOC values
MIL or battery-system warning lamp, possibly intermittent
Inability to read BMS live data or sporadic data dropouts on scan tool
Reduced charging or vehicle derate/limited power when BMS data unavailable
Loss of ancillary functions that rely on BMS data (remote charge scheduling, thermal management)
Communication-related DTCs set in other modules (gateway, charger, HV inverter)

What to check

Check battery pack main voltage is within expected range and stable during occurrences
Inspect fuses and relays providing power to the BMS; verify continuity and seating
Visually inspect BMS connector and mating harness for corrosion, bent pins, water, or damage
Wiggle-test harness/connectors while monitoring CAN/diagnostic data for intermittent changes (safe bench method)
Measure CAN_H/CAN_L idle voltages with a DMM (~2.5V common mode) and differential amplitude with scope
Scan vehicle for other related U/C codes (gateway or other control modules) and note freeze-frame

Signal parameters

CAN_H idle ~2.5 V common mode; CAN_L idle ~2.5 V common mode; differential peak ~2.0 V (vehicle-dependent)
Bus bit rate (e.g., 500 kbps, 250 kbps, 125 kbps) — confirm correct network speed
Message ID(s) used by BMS and expected update rate (Hz) for critical frames
Message interval/timestamp: look for dropped frames or increased gap vs expected rate
RX/TX error counter values and any bus-off status reported by gateway
Checksum/CRC validity and sequence counters in BMS messages (if applicable)

Diagnostic algorithm

Read and record all DTCs from the BMS and gateway; capture freeze-frame data and timestamps.
Reproduce the fault if possible while monitoring live CAN messages and BMS outputs with a capable scan tool or CAN logger.
Inspect physical connectors at BMS and intermediate harnesses; disconnect, inspect pins, clean and re-seat connectors. Replace any corroded terminals.
Verify BMS supply fuses/relays and measure supply voltage and ground at the BMS harness under static and dynamic conditions.
Check CAN bus wiring continuity and resistance between BMS and gateway/ECU; look for high resistance or intermittent opens using backprobe or harness isolation.
Use an oscilloscope to observe CAN_H/CAN_L signals during normal operation and when the fault occurs; look for noise, reflections, missing recessive/dominant transitions, or transients.
Verify termination resistors and look for multiple terminations or missing termination; measure differential resistance with ignition on (~60 ohm typical).
Inspect for mechanical damage, chafing points, water intrusion, or recent repairs near the harness route; repair damaged wiring with proper solder/crimp and insulation.
Check for module sleep/wake wiring/signals; verify that the BMS is being woken by the correct trigger and not unintentionally powering down.
If wiring/power/termination are good, check module software versions and service bulletins; reflashing or reprogramming may be required per manufacturer guidance.
If intermittent persists and wiring/power/software confirmed good, consider bench-testing or replacing the BMS module. Follow safe HV isolation and manufacturer safety procedures.
After repairs, clear DTCs and perform multiple drive or charge cycles to confirm fault does not return; re-scan all related modules.

Likely causes

Loose or corroded power/ground at BMS resulting in intermittent reset
Intermittent open/short on CAN_H or CAN_L wiring or connector
Damaged or water-ingressed BMS connector pins causing intermittent contact
CAN bus termination missing or intermittent (causing reflections/communication loss)
BMS entering unintended sleep mode or failing to wake due to software/config issue

Fault status

⚠️ Status

Intermittent loss of communication detected between vehicle network and Battery Management Module (BMS). Communication frames from the BMS are missing or intermittent; may cause other modules to report dependent communication faults.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.5-4 hours