Code
B2978
MITSUBISHI
B — Body
Body Control Module (BCM) General Fault
Views:
UK: 10
EN: 9
RU: 10
AI status
Completed
Completed
100%
Causes
- Loose, corroded or damaged BCM connectors or wiring
- Blown fuse or failed relay supplying BCM
- Low or unstable battery/charging system voltage
- Water ingress or physical damage to BCM
- Failed BCM internal electronics or software fault
- CAN/LIN communication errors or bus short/open
Symptoms
- One or more body systems inoperative or intermittent (interior lights, exterior lights, door locks, windows, wipers)
- Instrument cluster warnings or indicator lamp illumination
- Doors/locks behave erratically or remote/keyless entry not working
- Battery drain or no-crank condition related to body systems
- Stored communication-related DTCs (CAN/LIN) or multiple body system faults
What to check
- Connect a compatible scan tool and read all stored/pending DTCs and freeze frame data
- Check battery voltage at rest and with ignition ON (should be ~12.6 V at rest, 13.5–14.5 V when charging)
- Visually inspect BCM connectors and wiring for corrosion, bent pins, water intrusion or damage
- Verify BCM supply fuse(s) and relay(s) for continuity and proper operation
- Confirm good ground at BCM (measure voltage drop between ground and chassis while loading circuits)
- Check CAN/LIN bus health: presence of bus messages, termination resistances (~60 ohms across CAN high/low), and idle voltages (~2.5 V)
Signal parameters
- BCM supply voltage: 11–14.5 V (ignition ON)
- Battery/rest voltage: ~12.4–12.8 V (engine off, charged battery)
- CAN bus idle voltage: ~2.5 V on CAN_H and CAN_L differential ~0 V but CAN_H ~2.5 V, CAN_L ~2.5 V; differential ~2 V when active
- Termination resistance across CAN_H and CAN_L: ~60 ohms
- Ground resistance:
- LIN bus idle voltage: ~12 V (via pull-up) or specified by manufacturer
Diagnostic algorithm
- Record all DTCs and freeze frame data using a factory-level scan tool. Note any related U-codes or multiple body module faults.
- Clear codes and perform functional re-test to see if B2978 returns and under what conditions (ignition ON, accessory use, door operation).
- Check battery state and charging system. Low voltage can cause BCM faults — verify battery charge and alternator output.
- Perform visual inspection of BCM and harness: look for water, corrosion, damaged seals, rodent damage, pin corrosion, or signs of overheating.
- Verify fuses/relays supplying BCM: test for continuity and proper voltage at BCM power pins with ignition ON and OFF.
- Test grounds: measure voltage drop from BCM ground to chassis while exercising body functions; excessive drop indicates poor ground.
- Check communication buses: use oscilloscope or scan tool to confirm CAN/LIN traffic and proper voltages/termination. Isolate segments if necessary to find node causing bus fault.
- Back-probe supply and signal pins at BCM connector to confirm stable inputs/outputs while exercising functions (watch for intermittent faults when wiggle-testing harness).
- Inspect for aftermarket accessories or recent repairs that could have altered wiring; disconnect if suspect.
- If wiring, power and communications verify correct and fault persists, follow manufacturer procedure for BCM self-tests, software updates or reprogramming. Replace BCM only after confirming external causes have been eliminated and reprogramming can be performed.
- After repair or module replacement, clear codes and verify system operation over time; perform final road/test cycle and confirm no reoccurrence.
Likely causes
- Corroded/loose BCM ground or power connector
- Faulty BCM power supply fuse or relay
- Intermittent CAN/LIN bus communication fault (open/short/noisy)
- Water damage or connector contamination at BCM
- Failed BCM module requiring reprogramming or replacement
Fault status
Status
General BCM fault — internal error or detected fault in BCM power/communications/inputs causing body system malfunctions.
Repair difficulty: Medium
Diagnostic time: 1.0-3.0 hours
Similar codes
Your experience will help others
+100 karma for a short comment :)
Was this AI description helpful?
Your feedback helps improve AI descriptions.
👍 Like
0
👎 Dislike
0
Send to email