B0400

Code

Generic B — Body

Undefined Body Control Module Fault (B0400)

Brand: Generic

Type: B — Body

Views: UK: 16 EN: 24 RU: 19

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Faulty BCM internal hardware or software
Low or unstable battery voltage / poor battery connection
Blown fuse or defective relay supplying BCM
Corroded/loose BCM connector or wiring harness damage
CAN/LIN network communication errors or bus short/open
Failed or incompatible BCM software calibration

Symptoms

Multiple body functions fail or behave intermittently (lights, locks, windows, wipers)
Warning lamp or message related to body electronics or module fault
Inability to communicate with BCM using a scan tool
Parasitic battery drain or abnormal battery charging behavior
Vehicle not recognizing key/remote functions or central locking inoperative

What to check

Retrieve all stored/witness codes from BCM and other modules; record freeze frame and occurrence count
Check and record battery resting and cranking voltage; verify battery terminals and ground connections
Inspect BCM fuses and relays; verify presence of ignition and battery power at BCM fuse/relay outputs
Visually inspect BCM connector(s) for corrosion, bent pins, moisture or damage
Scan vehicle network for module presence and CAN/LIN error counters; check for lost modules
Perform wiggle test on wiring harness while monitoring for code or symptom changes

Signal parameters

Battery voltage at BCM (key off / key on / cranking)
Ground continuity/resistance between BCM ground pin and chassis/battery negative
CAN bus voltages (CAN_H, CAN_L idle ~2.5V) and differential voltage
LIN bus voltage levels and wake/sleep activity if applicable
Module response to diagnostic poll (ACK/NACK, timeouts)
Supply current draw of BCM (parasitic draw measurement)

Diagnostic algorithm

Retrieve and record all DTCs from BCM and other modules. Note occurrence counts and freeze data. Clear codes and see if B0400 returns with key cycles or vehicle operation.
Verify battery state of charge and inspect/clean battery positive/negative terminals and chassis grounds. Re-test after securing connections.
Check BCM power/ground fuses and relays. Back-probe power supply pins at BCM to confirm stable battery and ignition voltages with key off/on/crank.
Visually inspect BCM connectors and harness for corrosion, damaged insulation, pin mismatch or water intrusion. Repair or replace damaged connectors/wiring as required.
Use a scan tool to verify BCM is present on the network and monitor CAN/LIN traffic for errors, bus-off or module dropouts. Address any other module communication faults first (U-codes).
Measure CAN bus voltages and differential signal with oscilloscope or DMM; look for shorts, opens, or dominant bus states. Isolate sections of bus if needed.
Remove or disconnect recent aftermarket accessories that could feed or disturb vehicle networks; retest.
If wiring and supplies are good but the code persists, check for available BCM software updates or reprogramming procedures and perform reflash per manufacturer guidance.
If programming fails or diagnostics point to internal BCM faults after all checks, consider module replacement and perform required programming/coding to vehicle after replacement.
After any repair, clear codes and verify the fault does not return through normal vehicle operation and confirm all previously affected systems function correctly.

Likely causes

Poor battery/ground connections causing brownout of BCM
Intermittent connector pins to BCM due to corrosion or vibration
CAN bus short to voltage or ground preventing module communication
BCM firmware corruption after a failed reflash or update
Internal BCM component failure (logic/power supply)

Fault status

⚠️ Status

Body Control Module reported an undefined/internal fault or communication failure. Investigate power, grounds, network communications, connectors, software and BCM hardware integrity.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-3.0 hours