B1A42

Code

Generic B — Body

Left Rear Door Module CAN Bus Communication Fault

Brand: Generic

Type: B — Body

Views: UK: 13 EN: 9 RU: 9

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Page language: EN

Causes

Damaged or chafed CAN bus wiring (CAN H / CAN L) between LRDM and gateway
Corroded, loose or water-damaged connectors at the LRDM or splice points
LRDM power or ground fault (low supply voltage or intermittent ground)
Failed LRDM internal CAN transceiver or electronics
Bus short to battery or ground on CAN H or CAN L
Missing or incorrect CAN termination (open or shorted terminator)

Symptoms

Left rear door functions (power window, lock/unlock, courtesy lamp, mirror/puddle lamp) intermittent or inoperative
Driver information center or instrument cluster shows 'door open', 'communication' or specific LRDM warnings
Central locking or alarm behaves inconsistently for that door
Related DTCs for other modules or 'U' codes indicating bus problems may be stored
No LRDM modules listed or no heartbeat messages shown by a capable scan tool

What to check

Read all stored and pending DTCs and note freeze-frame data and timestamp
Check for other modules reporting CAN bus or communication-related DTCs (U-codes)
Visually inspect LRDM connector, pins and door harness especially through the door hinge boot
Check LRDM supply voltage and ground with key ON; confirm stable battery voltage (approx. 12 V)
Backprobe or measure CAN H and CAN L at LRDM connector for expected idle voltages and activity
Measure resistance between CAN H and CAN L with power off (expect ~60 Ω if two 120 Ω terminators in parallel)

Signal parameters

CAN bus idle voltages: CAN H ≈ 2.5 V, CAN L ≈ 2.5 V
Dominant state typical voltages: CAN H ≈ 3.5 V, CAN L ≈ 1.5 V (differential ≈ 2.0 V)
Termination resistance across CAN H and CAN L ≈ 60 Ω (two 120 Ω terminators in parallel)
Common CAN speeds: 500 kb/s or 250 kb/s (vehicle-specific)
Module supply: typical battery voltage ~11–14.5 V with engine off/on respectively
LRDM should transmit periodic status/heartbeat messages (frequency varies by manufacturer)

Diagnostic algorithm

Use a scan tool to read codes on all modules. Confirm B1A42 and note any U-codes or additional body module faults.
Attempt to communicate with the LRDM via a dealer-level or capable scan tool. Note whether the module responds and whether live data/heartbeat is present.
Perform a visual inspection of the left rear door harness, boot, and connector for corrosion, broken wires, paint overspray, or water intrusion. Pay special attention to the hinge area where wires flex.
With connector mated, measure supply voltage and ground at LRDM pins (Key ON). Verify stable battery voltage and good ground continuity. Repair any supply/ground issues before further testing.
Backprobe CAN H and CAN L at the LRDM connector with scope or multimeter. With key ON, verify idle voltages (~2.5 V) and that there is message activity (using an oscilloscope or CAN bus analyzer).
If no activity, measure resistance across CAN H and CAN L with power OFF to confirm proper termination (~60 Ω). If open or shorted, trace and repair wiring/terminators.
If voltages are abnormal (stuck high/low), isolate by disconnecting the LRDM and rechecking bus behavior. If disconnecting the LRDM restores normal bus operation, suspect LRDM internal fault.
Wiggle or flex the door harness and monitor the bus for intermittent faults. Repair broken wires or damaged conductor pairs (patch or harness replacement as required).
Inspect and repair any prior corrosion or water damage. Clean/replace corroded connectors and apply dielectric grease where appropriate.
If wiring and power/ground are good but the LRDM fails to communicate, replace the LRDM or have it bench-tested/reprogrammed per manufacturer procedures.
After repairs, clear DTCs and perform functional test cycles (lock/unlock, window, lamp) and re-scan to ensure code does not return. Road test to confirm intermittent issues are resolved.

Likely causes

Open, shorted or intermittent CAN H / CAN L wiring in left rear door harness (hinge area)
Corroded/wet connector at left rear door module
LRDM has lost battery feed or ground connection
Failed LRDM CAN transceiver