B1A7C — Body Control Module CAN Bus Communication Fault

Causes

• Open or short in CAN High or CAN Low wiring
• Poor connector contact, corrosion, bent pins, or water ingress at BCM or network connectors
• Failed or intermittent BCM power supply or ground
• Missing or incorrect CAN bus termination (failed/missing resistor)
• Another module on the bus driving the line (short to Vbatt or ground) or sending malformed frames
• Faulty BCM internal transceiver or logic

Symptoms

• Multiple control modules show intermittent or no communication on scan tool
• Loss or intermittent operation of BCM-controlled features (lights, door locks, wipers, HVAC, etc.)
• Warning lights (CAN communication, body system errors) illuminated
• Vehicle may enter limp or safe mode for affected systems
• Intermittent drivability or starting issues if key/body messages are missing

What to check

• Connect a scan tool; record active/pending U/B codes and module list. Note freeze frame data and occurrence conditions
• Verify battery voltage and BCM power/grounds with key on; low or unstable battery can cause comms faults
• Visually inspect BCM and harness connectors for corrosion, bent pins, water entry, or damage
• Check for other modules reporting lost communication (helps isolate segment or node)
• Measure CAN bus idle voltages (CAN H and CAN L) and differential with meter or scope
• Check bus termination resistance between CAN H and CAN L (~60 Ω) with ignition off

Signal parameters

• CAN bus idle (recessive): CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (each ± ~0.5 V depending on vehicle)
• CAN bus dominant state: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ≈ 2.0 V typical)
• Bus termination: approximately 60 Ω measured between CAN_H and CAN_L (two 120 Ω terminators in parallel)
• Common data rates: 125 kbps, 250 kbps, 500 kbps (verify vehicle-specific rate with scan tool)
• Low-frequency intermittent faults may show voltage spikes, missing frames, or error counters increasing on scope

Similar codes