Code
B29D7
MITSUBISHI
B — Body
Body Control Module — CAN Communication Fault
Views:
UK: 7
EN: 12
RU: 10
AI status
Completed
Completed
100%
Causes
- Damaged or chafed CAN wiring harness
- Faulty/loose connector at BCM or other network module
- Failed BCM (internal communications circuitry)
- Missing or incorrect CAN bus termination (open/shorted resistors)
- Short to battery or ground on CAN_H or CAN_L
- Low system voltage, blown fuse, or poor BCM ground
Symptoms
- Multiple body functions (locks, lights, HVAC, wipers) inoperative or intermittent
- Inability to communicate with BCM using diagnostic tool (scanner shows module not responding)
- Multiple related modules report communication or bus-off errors
- Illumination of communication or master warning lamp
- Intermittent faults that occur with movement, temperature change, or moisture exposure
What to check
- Read and record all stored/active DTCs and freeze-frame data with a capable scan tool
- Note whether the fault is current, intermittent, or history and any accompanying U-codes
- Visually inspect BCM area and CAN wiring for damage, chafing, water ingress, or corroded connectors
- Verify BCM power and ground circuits (voltage at supply pin, continuity to chassis ground) and check related fuses
- Measure DC voltages on CAN_H and CAN_L with ignition ON (see signal parameters)
- With ignition off, measure resistance between CAN_H and CAN_L at two points on the network (expected ~60 ohms with two 120-ohm terminators)
Signal parameters
- CAN_H voltage (idle, ignition ON): approximately 2.5–3.5 V (rests near 2.5 V differential when idle; CAN_H ≈ 2.5–2.9 V)
- CAN_L voltage (idle, ignition ON): approximately 1.5–2.5 V (CAN_L ≈ 2.1–2.5 V)
- Differential voltage (CAN_H − CAN_L) during data: ±1.5–3.5 V pulses
- Termination resistance (power OFF) between CAN_H and CAN_L: ≈ 60 Ω (two 120 Ω terminators in parallel)
- Short-check: infinite/open expected to battery on open circuits; low ohms indicates short to battery/ground
Diagnostic algorithm
- Connect a scan tool and record DTCs, their status (continuous/intermittent), and any U-codes. Note which modules do not respond.
- Inspect BCM connectors and nearby harnesses for physical damage, corrosion, or water; secure loose connectors. Address visible issues and clear codes to retest.
- Verify BCM supply and ground: measure battery voltage at BCM power pin with ignition ON and check ground continuity to vehicle chassis. Replace fuse or repair supply/ground faults as needed.
- With ignition ON, measure static voltages on CAN_H and CAN_L at the BCM connector and another known good node. Compare to expected values in signal parameters.
- With ignition OFF, measure resistance between CAN_H and CAN_L at two network locations to confirm correct termination (~60 Ω).
- Check for shorts: measure resistance from each CAN line to chassis ground and to battery positive. A low reading indicates a short; isolate and repair wiring or component causing the short.
- If voltages and termination are correct, use an oscilloscope or CAN bus analyzer to observe waveforms and error frames during key-on and when the fault is present. Look for noise, missing dominant/recessive transitions, or bus-off conditions.
- Isolate the faulty segment by disconnecting suspected modules or aftermarket devices one at a time and rechecking the bus until normal communication returns.
- If wiring and external modules check good and the problem follows the BCM, consider BCM reprogramming or replacement per manufacturer service procedures. Replace only after verifying root cause and re-initializing/reprogramming as required.
- After repairs, clear all codes, verify communication with the BCM and related modules, and perform a functional test/road test to ensure the fault does not return.
Likely causes
- Poor BCM power/ground or blown fuse preventing normal CAN transceiver operation
- Open or short in CAN_H/CAN_L between BCM and main gateway/module
- Faulty BCM transceiver or internal BCM failure
- Corroded or loose connector at BCM or a nearby junction
- Aftermarket accessory placed on CAN bus causing bus errors
Fault status
Status
Communication fault detected on the Body Control Module CAN network. The BCM is reporting loss or corruption of messages on the vehicle CAN bus; related body systems may be affected and the module may be non-responsive to a scan tool.
Repair difficulty: Medium
Diagnostic time: 1.0-3.0 hours
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