B1A02

Code

Generic B — Body

Body Control Module CAN Bus Communication Fault

Brand: Generic

Type: B — Body

Views: UK: 13 EN: 17 RU: 12

AI status

Completed

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Page language: EN

Causes

Open or shorted CAN wiring (CAN_H or CAN_L) between BCM and other modules
Loose, corroded, or improperly seated connector at BCM or other modules
Faulty BCM or another node flooding/breaking the CAN bus
Terminator resistor missing, shorted, or incorrect value
Low system battery voltage or poor ground connections causing intermittent comms
Aftermarket electronics or wiring splices interfering with CAN signals

Symptoms

Multiple modules show 'communication lost' or 'no data' messages on scan tool
Intermittent or complete loss of BCM-controlled functions (lights, locks, instrument cluster messages)
Warning lights on dash (CAN/network or multiple unrelated systems)
Random module reboots or erratic behavior of networked systems
No CAN traffic or severely reduced traffic when monitoring with a scan tool or oscilloscope

What to check

Scan for additional U or B codes and note which modules report lost communication
Visually inspect BCM connectors, harnesses, and nearby junctions for damage, corrosion, or aftermarket splices
Check battery voltage and main ground connections; ensure stable supply during testing
Backprobe CAN_H and CAN_L at the BCM and at another known-good module to compare voltages
Measure bus termination resistance across CAN_H and CAN_L with power off (should be ~60 ohms for two 120Ω terminators in parallel)
Use an oscilloscope to view CAN_H and CAN_L waveforms for dominant/recessive levels, noise, and message timing

Signal parameters

CAN bus nominal bit rates commonly 125 kb/s, 250 kb/s, or 500 kb/s — verify vehicle-specific rate
Idle voltages: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V; dominant state: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (approximate values)
Differential voltage (CAN_H - CAN_L) ~2.0 V when dominant
Bus termination resistance across CAN_H and CAN_L ≈ 60 ohms (two 120Ω in parallel)
Error frame/CRC errors and retransmissions visible on oscilloscope or CAN analyzer
Message IDs expected from BCM and peer modules; note missing or corrupted frames and timing irregularities

Diagnostic algorithm

Retrieve freeze frame and full DTC list; record which modules report communication faults and timestamps if available.
Verify vehicle battery voltage (12.4–12.8 V static or >13.5 V with engine running) and check main and BCM ground points for tightness and corrosion.
Visually inspect BCM connector and CAN harness for damage, corrosion, chafing, or aftermarket taps. Repair any obvious harness issues.
With key on (engine off), backprobe CAN_H and CAN_L at BCM and at a second module. Compare voltages to expected idle levels and to each other.
With a scan tool or CAN bus analyzer, monitor live CAN traffic. Note bit rate, presence of normal messages, and any repeating error frames or bus-off conditions.
Measure termination resistance with battery disconnected: verify ~60 ohms across CAN_H and CAN_L at multiple points along the bus.
If abnormal voltages/noise, use an oscilloscope to inspect waveforms for dominant/recessive levels, ringing, and short pulses. Trace wiring to locate shorts or high-impedance joints.
If a single module appears to be transmitting invalid frames or pulling the bus low, disconnect that module (or its CAN connector) and observe whether the fault clears.
If wiring and peer modules check good, consider reflashing or replacing BCM only after verifying parts compatibility and performing post-repair network learn procedures as required by manufacturer.
After repairs, clear codes and verify normal CAN traffic and restoration of all lost module communications; road test if necessary.

Likely causes

Intermittent or high-resistance connector at the BCM or nearby junction (most common)
Short between CAN_H and CAN_L or to battery/ground at a damaged wire harness section
Failed termination resistor (open or doubled termination)
Another control module stuck and flooding the bus with error frames
Vehicle battery low voltage or poor chassis ground causing signal distortions

Fault status

⚠️ Status

BCM detected CAN bus communication fault — missing or corrupted messages on CAN network. Check wiring, terminations, module connectivity, and bus health.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1-3 hours