B3020

Code

Generic B — Body

Body Control Module Communication Fault

Brand: Generic

Type: B — Body

Views: UK: 7 EN: 12 RU: 9

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Page language: EN

Causes

Low or missing battery voltage
Blown fuse or fusible link for BCM power/ignition circuits
Poor or corroded ground connections to BCM
Damaged wiring or open/shorted CAN/LIN/serial communication lines
Faulty termination resistor on CAN bus
Water ingress or connector corrosion at BCM

Symptoms

Warning lights illuminated (BCM, communications, or check engine lamp)
Loss of remote/keyless functions, door locks, lighting, or accessories
Intermittent or permanent failure of body functions (windows, wipers, interior lights)
No response from BCM-controlled systems, or multiple modules reporting communication errors
Speedometer/cluster or infotainment glitches if network messages are missing

What to check

Read all modules with a capable scan tool; record freeze frame and additional U/B codes
Verify battery voltage with key on and engine off (should be ~12.0–12.6 V) and charging voltage with engine running (13.5–14.8 V)
Inspect BCM fuses and fusible links; verify presence of battery and ignition feed voltages at BCM pins
Inspect and clean BCM connectors and ground points; check for corrosion, bent pins, or water intrusion
Visually inspect CAN/LIN wiring for damage, chafing, or rodent bites along harness
Check for proper CAN bus termination (120 ohm across CANH and CANL at bus ends)

Signal parameters

Battery system voltage: ~11.5–14.8 V (engine running ~13.5–14.8 V)
CAN bus idle voltages: CANH ≈ 2.5 V, CANL ≈ 2.5 V (dominant state CANH ≈ 3.5 V / CANL ≈ 1.5 V)
CAN bus differential nominal: ~0 V (recessive) to ~2 V (dominant)
Typical CAN data rates: 125 kb/s, 250 kb/s, or 500 kb/s depending on vehicle
LIN bus idle: near battery voltage with occasional pulls low during communication (varies by vehicle)
Module supply current asleep: typically low microamp range (check manufacturer spec)

Diagnostic algorithm

Confirm the code: clear DTCs, then reproduce or wait for code to reappear while monitoring networks with a scan tool.
Read all stored and pending codes from all modules; note any related U-codes or multiple module losses.
Verify battery and charging system voltages; recharge or replace weak battery before further testing.
Inspect fuses and power feeds to BCM. Backprobe BCM power/ignition pins to confirm correct voltages with key on/off.
Check BCM ground(s): measure continuity to chassis and battery negative; clean/tighten ground connections.
Visually inspect BCM connector for corrosion, bent pins, or water damage; repair or replace damaged connectors.
Using a multimeter/oscilloscope, measure CANH/CANL at the BCM: verify idle voltages, presence of differential data while bus active, and absence of shorts to battery/ground.
Check for proper termination resistance (≈60 ohm measured across CANH/CANL at one point if both 120Ω resistors present; each end ≈120Ω).
Wiggle test harness while monitoring network for interruptions; isolate segment with fault by disconnecting suspected modules one at a time.
If wiring and power/ground are good and communication still fails, compare BCM software/calibration level to other modules; reprogram/update if recommended by manufacturer.
If substitution is necessary, confirm fault follows module before replacing the BCM; when replaced, program/configure the new BCM per manufacturer procedure.
After repairs, clear codes and perform functional verification of all BCM-controlled systems and road test if required.