B3603

Code

Generic B — Body

Body Control Module CAN Bus Communication Fault

Brand: Generic

Type: B — Body

Views: UK: 12 EN: 15 RU: 11

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Page language: EN

Scan for all active, pending and historic DTCs; note any U-codes and freeze frame data
Verify BCM has proper battery feed and fused power; check grounds with low-resistance meter
Visually inspect BCM connector, pins, and CAN wiring for damage, corrosion, or aftermarket splices
Back-probe CAN_H and CAN_L at the BCM and other network nodes with a DMM and/or scope
Measure CAN idle voltages and differential, and check termination resistance across bus
Disconnect suspected aftermarket modules or accessories and re-check bus

CAN bus common idle (recessive): CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (both near midpoint)
CAN bus dominant state: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V; differential ≈ 2 V
Nominal bus termination: approx. 60 Ω total between CAN_H and CAN_L (two 120 Ω end resistors in parallel)
Typical data rates: 125 kb/s, 250 kb/s, or 500 kb/s depending on vehicle network
Expected steady stream of message IDs and message intervals seen with a known-good scan tool or CAN sniffer

Record vehicle history and conditions when the code set. Retrieve all codes and freeze-frame data with a capable scan tool.
Attempt to communicate with BCM and other modules using a scan tool. Note which modules respond and which are missing.
Visually inspect BCM connector, harness, fuses and grounds. Repair any obvious damage or corrosion before further testing.
Verify BCM supply voltage and ground continuity. Repair poor supply/ground connections if found.
Back-probe CAN_H and CAN_L at the BCM with the ignition ON. With a DMM check idle voltages; with an oscilloscope check waveform shapes, differential voltage, and presence of expected message activity.
Check termination resistance between CAN_H and CAN_L (engine off). Expect about 60 Ω; a very low or infinite reading indicates short or open/removed termination.
Isolate the fault by disconnecting sections or suspected modules one at a time while monitoring the bus — this can reveal a failing node that drags the bus down.
If wiring and the rest of the bus test OK but the BCM does not participate, consider BCM internal failure. Confirm with known-good BCM or bench testing if available, taking care with vehicle security/immobilizer procedures.
Check for technical service bulletins (TSBs) or software updates before replacing parts. Reprogram or configure replacement BCM per manufacturer procedures and verify all modules communicate after repair.
Clear DTCs and road-test to confirm repair. Re-scan and verify no recurrence.