B3478

Code

Generic B — Body

Body Control Module Communication Fault

Brand: Generic

Type: B — Body

Views: UK: 9 EN: 13 RU: 11

AI status

Completed

ready

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Page language: EN

Causes

Faulty Body Control Module (BCM) or internal module error
Damaged, corroded, or loose connectors at BCM or network nodes
Broken, shorted, or high-resistance CAN/LIN/data bus wiring
Faulty termination resistor(s) or incorrect bus topology
Power or ground fault at the BCM (intermittent supply or poor ground)
One or more other modules on the bus misbehaving (bus-off, flooding messages)

Symptoms

One or more body systems not responding (locks, lamps, wipers, windows)
Intermittent or constant warning lights on dash related to body functions
Multiple related DTCs appearing for other modules (loss of communication)
Remote keyless entry, alarm, or immobilizer may not operate reliably
Instrument cluster messages missing or incorrect
Possible no-crank/no-start if BCM controls starter inhibit

What to check

Read all stored and pending DTCs and note freeze-frame and timestamp data
Check for related U-codes (lost communication with other modules) and multiple body codes
Visually inspect BCM connector(s) and nearby wiring for corrosion, water, or damage
Verify BCM power and ground voltages with key on and engine off
Measure CAN_H and CAN_L voltages at the BCM connector and at several nodes (Idle ~2.5 V each)
Check bus termination: measure resistance across CAN_H and CAN_L (approx. 120 ohms end-to-end)

Signal parameters

CAN_H idle ~2.5 V, CAN_L idle ~2.5 V; dominant state shows CAN_H > CAN_L by ~1.5–3.0 V
Differential voltage during dominant state ≈ 1.5–3.0 V
Bus termination resistance ~120 Ω across CAN_H and CAN_L (vehicle-specific)
Message IDs and intervals: expected periodic messages from BCM (check scan-tool list)
Error counters: CAN error warning, bus-off conditions, transmit/receive error counts
LIN bus typical idle voltage per manufacturer (check vehicle spec)

Diagnostic algorithm

Confirm code presence and record freeze-frame data; scan other modules for related U/B codes
Clear codes and attempt to re-run the failure; note whether B3478 returns and under what conditions
Inspect BCM connector, pull back boot/loom, look for corrosion, bent pins, water ingress and repair any damage
Verify battery voltage at BCM and measure main power feed and ground continuity to chassis
Measure CAN_H and CAN_L voltages at the BCM with ignition on; compare to a known-good node and expected idle values (~2.5 V)
Backprobe CAN wires while observing with an oscilloscope or scan tool to confirm valid differential waveform and message traffic
Check termination resistance across CAN_H/L (engine off, battery disconnected if required by procedure)
If one node dominates (bus flooding or bus-off), identify that module by disconnecting nodes one at a time per manufacturer procedure and rechecking the bus
Repair any wiring faults, corrosion, or poor grounds. Secure connectors and apply dielectric grease where appropriate
If wiring and power/grounds are good, check for available BCM software updates and reflash per TSBs; perform module reinitialization if replacement occurred
If BCM is still suspected after all wiring/software checks, consider module replacement or professional bench testing with a known-good BCM
After repair, clear codes and verify correct operation under the same conditions that produced the fault

Likely causes

Loose or corroded connector at the BCM
Open, short, or high resistance in CAN_H or CAN_L wiring near BCM
Poor ground or low battery/ignition supply to BCM
Another module on the network has gone bus-off and prevents normal communication
Software mismatch after module replacement or failed module reflash

Fault status

⚠️ Status

Body Control Module communication fault detected — messages on the vehicle data bus are missing, corrupted, or causing a communication error between the BCM and other control modules.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.5-4 hours