B2257

Code

Generic B — Body

Body Control Module (BCM) - Communication Bus Fault

Brand: Generic

Type: B — Body

Views: UK: 7 EN: 13 RU: 7

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Damaged or chafed wiring on the body communications bus (CAN/LIN)
Poor or corroded connector terminals at BCM or other modules
Short to battery or ground on CAN_H or CAN_L / LIN signal
Open circuit in bus wiring or broken splice
Missing or incorrect bus termination (open or shorted resistors)
Faulty BCM or another control module flooding the bus with errors

Symptoms

Multiple body systems nonfunctional or intermittent (doors, lighting, wipers, remote lock)
Warning lights or lamp on dash related to body electronics or communication
Scan tool cannot communicate with BCM or shows limited data/‘no response’
Erratic operation of modules that depend on BCM messages
Stored manufacturer or U-codes related to control module communication (U0073, U0100, etc.)

What to check

Connect a capable scan tool; read and record all BCM and network-related codes and freeze frame data
Visually inspect BCM connector(s) and the bus wiring for damage, corrosion, or rodent chew
Check battery voltage and charging system (12.4–14.7 V) and ensure stable power during tests
With key off, measure resistance between CAN_H and CAN_L at the bus ends to confirm termination (~60 ohms total)
Backprobe communication lines at the BCM and at a downstream module to confirm presence of signals
Check for presence of aftermarket devices on the bus and disconnect them

Signal parameters

Typical CAN bus idle (recessive): CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (each referenced to chassis ground)
Typical CAN dominant state: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ~2 V)
Termination resistance: ≈ 60 Ω measured across CAN_H and CAN_L (two 120 Ω terminators in parallel)
LIN idle: single wire near battery voltage when idle; dominant pulls toward ground (refer to vehicle-specific values)
Battery voltage during tests: 12.4–14.7 V (no large drops during communication checks)

Diagnostic algorithm

Record all codes and module availability with a professional scan tool; note network status and module list. 2) Perform visual inspection of BCM connectors, harness, and grounding points; repair any damage. 3) Verify battery/charging system health; clear codes and attempt re-scan. 4) Measure termination resistance across CAN_H and CAN_L with ignition off; expect ≈60 Ω. 5) With ignition on, backprobe CAN/LIN at BCM and a known-good module; confirm expected idle voltages and presence of transitions using a digital multimeter (DC) and preferably an oscilloscope for waveform inspection. 6) If one wire is shorted to power/ground or open, isolate by disconnecting sections or modules and rechecking bus signals (disconnect suspected modules one at a time). 7) Remove or disable aftermarket accessories and re-test. 8) If wiring and external modules check good but bus traffic is absent or a single module dominates errors, follow manufacturer procedure to update/reflash BCM software; reprogram where required. 9) Replace BCM only after isolating fault to BCM; after replacement, program and verify function, clear codes, and road test. 10) Document findings and repairs, and confirm no other related codes remain.

Likely causes

Open or short in CAN/LIN wiring between BCM and main junction connector
Loose or corroded BCM connector pins
Short to ground or +12V on CAN_H or CAN_L due to damaged harness or aftermarket equipment
Defective BCM failing to drive/receive bus signals
Missing/failed termination resistor(s) causing reflections and errors

Fault status

⚠️ Status

BCM detected communication bus fault — messages missing, corrupted, or bus not responding. Inspect wiring, connectors, termination, and related modules.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-3.0 hours