B1A4A

Code

Generic B — Body

Body Control Module CAN bus communication fault

Brand: Generic

Type: B — Body

Views: UK: 9 EN: 15 RU: 13

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Page language: EN

MIL or warning lamp related to communications
Multiple modules reporting intermittent or permanent loss of communication
BCM-controlled systems intermittent or nonfunctional (lights, locks, windows)
Reduced or no functionality of vehicle networks (no immobilizer, no instrument cluster data)
Possible battery drain if a module is stuck transmitting

Read and record all current and pending DTCs and freeze frame with a scan tool
Check battery voltage (key off and key on) and battery state of charge
Visually inspect BCM connector, wiring harness, and adjacent harnesses for damage, corrosion or pin pullout
Check fuses and power/ground circuits for the BCM
Verify presence of correct CAN termination (measure resistance between CAN_H and CAN_L with ignition off)
Use scan tool to monitor CAN message traffic and check for module presence and message IDs

CAN bus idle (recessive): CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (differential ≈ 0 V)
CAN bus dominant: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ≈ 2 V)
Expected termination: ~60 Ω from CAN_H to battery positive bias and ~60 Ω from CAN_L to ground commonly measured as ~120 Ω across CAN_H–CAN_L when both terminations present
Typical data rates: 125 kb/s, 250 kb/s, 500 kb/s or 1 Mb/s depending on vehicle — verify expected baud rate
Oscilloscope should show clean square-ish CAN voltage waveforms; excessive noise, clipped levels, or bus-off conditions indicate wiring/module faults

Retrieve all DTCs from BCM and other modules; note time stamps and any related U-codes. 2) Check battery voltage (12.4–12.8 V resting, >12 V with key on) and verify solid chassis and engine grounds. 3) Perform a visual inspection of BCM connector and harness for corrosion, bent pins, water intrusion, or damage; repair any issues. 4) With ignition off, measure resistance across CAN_H and CAN_L at the BCM connector — expect ~60–120 Ω depending on bus architecture; an open or very low resistance indicates missing/shorted termination. 5) With a two-channel scope or CAN probe, observe CAN_H and CAN_L with ignition on and engine off/idle; look for proper voltage levels, differential signals, noise, or a bus-off state. 6) If a short/short-to-power is suspected, isolate segments by disconnecting downstream modules one at a time while watching for the fault to clear to locate the offending branch. 7) Check power and ground pins at BCM for proper voltage under load; repair high-resistance connections. 8) Repair or replace wiring, connectors, terminals, or external module found faulty. 9) If wiring and external modules are good, consider BCM replacement or reprogramming only after confirming harness integrity and battery/ground conditions. 10) After repairs, clear codes, perform functional verification and road test while monitoring CAN traffic to ensure the fault does not return.