B1A7A

Code

Generic B — Body

Body Control Module - CAN Bus Message Missing

Brand: Generic

Type: B — Body

Views: UK: 18 EN: 24 RU: 22

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Page language: EN

Causes

Faulty sending module (module that should transmit the message)
Open, short, or high-resistance wiring on CAN_H or CAN_L
Corroded or loose connectors at modules or junctions
Missing or failed CAN bus termination (incorrect resistance)
Low battery or poor power/ground to sending or receiving modules
Software/firmware mismatch or module not programmed

Symptoms

BCM reports module communication fault or disables related functions
Loss or intermittent operation of functions controlled by the missing message (doors, lighting, climate, etc.)
One or more modules not present/communicable on a scan tool
Warning lamps or DTCs for multiple modules
Intermittent faults that may clear/reappear

What to check

Connect a capable scan tool and confirm B1A7A and check for other communication codes
Note which module/message ID is reported missing and review module list (which modules appear on the bus)
Inspect battery state and clean/secure battery terminals; ensure stable vehicle voltage during testing
Visually inspect CAN wiring harness, branch points, and connectors for damage, corrosion, or water intrusion
Measure CAN bus termination: expected ~60 Ω across CAN_H and CAN_L (two 120 Ω in parallel)
Measure idle voltages: CAN_H ~2.5 V, CAN_L ~2.5 V (recessive); dominant states typically ~3.5 V and ~1.5 V respectively

Signal parameters

Expected CAN IDs: manufacturer-specific (check wiring/diagnostic data for exact ID(s))
Message period: manufacturer-defined (commonly periodic frames from 10 ms to 1000 ms depending on signal)
Bus voltage (recessive): CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V
Bus voltage (dominant): CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V
Termination resistance: ≈ 60 Ω measured across CAN_H and CAN_L with key off
Typical data rates: 500 kbps or 250 kbps commonly used (verify vehicle-specific rate)

Diagnostic algorithm

Retrieve freeze frame and all stored DTCs from all control modules; record which module or message ID is reported missing.
Verify battery voltage is within specification and ensure stable supply during testing (low voltage can cause modules to fail to transmit).
Confirm which modules appear on the CAN network using a scan tool. If the sending module is completely absent, focus on power/ground and its local wiring.
Visually inspect and gently wiggle harnesses and connectors along the CAN circuit, paying attention to areas that flex, near door hinges, or where water can enter.
With key on (and following safe procedures), measure CAN_H and CAN_L voltages at the BCM and at the suspected sending module. Compare to expected idle voltages.
Measure termination resistance across CAN_H and CAN_L at multiple points. Replace or repair termination if out of spec (~60 Ω).
Use an oscilloscope or CAN analyzer to capture the bus waveform while the vehicle is awake. Look for missing IDs, corrupted frames, excessive errors, incorrect bit timing, or dominant stuck states.
If a specific module is suspected, disconnect that module (if safe and allowed) to see if bus errors change; conversely, reconnect or swap with a known-good module if available.
Check and repair any damaged wiring or corroded connectors. Repair splices per OEM repair methods; avoid using excessive solder without proper protection.
Reprogram/reflash modules if software mismatch or known software bug is suspected (follow OEM tooling and procedures).
After repairs, clear codes, perform a key cycle and road test to verify the message is received and the DTC does not return.
If all wiring and other modules check good and the message is still missing, consider BCM replacement as last resort following proper configuration and coding procedures.