Code
B3420
HUMMER
B — Body
Body Control Module CAN Bus Communication Fault
AI status
Completed
Completed
100%
Causes
- Open or short in CAN bus wiring (CAN High/CAN Low)
- Poor or corroded ground or battery power to BCM
- Failed or intermittent BCM
- Faulty or missing termination resistors (improper bus termination)
- Other module on the bus causing bus errors or bus-off condition
- Aftermarket electronics or repeater modules placed on CAN bus
Symptoms
- Multiple modules show communication faults or do not respond to scan tool
- Instrument cluster warnings (CAN, Service Vehicle Soon, or module message)
- Loss of body-related functions (locks, lighting, wipers, etc.)
- Intermittent operation of electrical accessories
- No BCM data or incomplete data visible with scan tool
- Possible vehicle starting issues if gateway/ECM communications affected
What to check
- Retrieve freeze frame and DTCs from all modules with a capable scan tool
- Confirm ignition voltage and battery condition (low voltage can cause comm faults)
- Visually inspect BCM connectors, pins, and nearby wiring for damage or corrosion
- Measure CAN High and CAN Low voltages with key on, engine off
- Check resistance between CAN High and CAN Low with power off (approx. 60 ohms total for two 120Ω terminators)
- Scan for other U-codes (bus off or lost communication) to identify affected network
Signal parameters
- CAN_H idle ≈ 2.5 V (can rise to ~3.5 V during dominant bits)
- CAN_L idle ≈ 2.5 V (can drop to ~1.5 V during dominant bits)
- Differential voltage (CAN_H − CAN_L) ≈ 0 V idle, ≈ 2 V dominant
- Typical data rates: 125 kb/s, 250 kb/s, or 500 kb/s depending on network
- Termination resistance ≈ 60 Ω measured across CAN_H and CAN_L (with power off)
- Error counters and bus-off status available from modules via scan tool
Diagnostic algorithm
- Use a professional scan tool to read all stored/active DTCs and network status from BCM and gateway; note time stamps and related U-codes.
- Verify battery voltage ≥ 12.4 V and good ground connections. Re-test after charging or securing battery connections.
- Inspect BCM connector and harness for water intrusion, corrosion, bent pins, or repair splices. Repair as needed.
- With ignition on, measure CAN_H and CAN_L at BCM connector and at gateway/module connectors. Compare to expected signal parameters.
- With vehicle power off, measure resistance between CAN_H and CAN_L at multiple network points (expect ≈ 60 Ω across entire bus).
- If differential/level measurements are abnormal, isolate sections of the bus: disconnect connectors or modules one at a time while monitoring bus health to find the faulty segment or module.
- Use a scope or CAN data-logger to observe waveform quality, bus noise, and message traffic. Look for repeated error frames, dominant stuck bits, or missing IDs.
- Check for any modules reporting bus-off or high error counters; cycle power to that module or remove it from the bus to see if the network recovers.
- Remove or disable aftermarket devices and re-check communication.
- If wiring and other modules test good and BCM is suspected, confirm correct calibration/software level and replace BCM only after verifying need; program/configure replacement BCM per factory procedures.
- Clear codes and verify repair by confirming stable CAN traffic and no recurrence of B3420 under normal operating conditions.
Likely causes
- Damaged CAN wiring between BCM and gateway/other modules
- Loose, corroded, or missing ground or battery feed to BCM
- Failed BCM or another module forced into bus-off
- Bad or missing CAN bus termination (open/short across network)
- Recent repairs or accessories installed that tapped into CAN
Fault status
Status
BCM reports a CAN bus communication fault — expected CAN messages missing or communication degraded between BCM and other modules.
Repair difficulty: Medium
Diagnostic time: 1-4 hours
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Repair manuals
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138
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