B1C9D — Body Control Module CAN Bus Communication Fault

Causes

• Open or shorted CAN wiring (CAN-H or CAN-L) between BCM and network
• Corroded, loose, or damaged connectors at the BCM or harness splice
• Missing or incorrect termination resistor(s) or improper bus topology
• Blown fuse, incorrect BCM supply or ground issues
• Intermittent short to battery or ground (wiring chafe, water intrusion)
• Faulty BCM or other module transmitting invalid CAN frames

Symptoms

• BCM-controlled functions intermittent or inoperative (lighting, locks, wipers)
• Multiple modules report communication or timeout DTCs
• Instrument cluster warnings about network/BCM
• No CAN messages from the BCM when monitoring with a scan tool
• Random faults, fuse blow, or battery drain in some cases
• Possible vehicle immobilizer/start problems if BCM interfaces with security systems

What to check

• Scan network with a capable scan tool: read DTCs from BCM and other modules, note freeze frame and event counts
• Verify vehicle battery voltage and BCM fuse(s) are OK
• Visually inspect BCM connector, harness, and nearby splices for corrosion, damage or water ingress
• Backprobe CAN-H and CAN-L at BCM and at other network nodes; measure idle voltages with key on (expected ~2.5 V each, recessive)
• Measure differential voltage during bus activity (expect ~1.5–2.0 V differential when dominant)
• Check termination resistance across CAN-H and CAN-L with power off (approx. 60 Ω for two 120 Ω terminators in parallel)

Signal parameters

• CAN-H idle ≈ 2.5 V, CAN-L idle ≈ 2.5 V (recessive)
• Dominant state: CAN-H ≈ 3.5 V, CAN-L ≈ 1.5 V (differential ≈ 1.5–2.0 V)
• Bus differential idle ≈ 0 V, differential during bits ≈ 1.5–2.0 V
• Expected bit rate: commonly 250 kb/s or 500 kb/s (verify vehicle-specific)
• Termination resistance (power off) ≈ 60 Ω across CAN-H/CAN-L for typical dual-terminator networks
• Message frequency and message IDs from BCM (verify expected IDs and update rate with OEM data)

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