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Home / DTC / B240B — BCM Data Bus 'B' Communication Fault

B240B — BCM Data Bus 'B' Communication Fault

Detailed page for trouble code B240B.

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Code

B240B

Generic B — Body

BCM Data Bus 'B' Communication Fault

Brand: Generic
Type: B — Body
Views: UK: 2 EN: 8 RU: 3
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Page language: EN

Causes

  • Open, shorted or corroded wiring/connector on Data Bus B lines
  • Poor BCM power or ground(s)
  • Failed or intermittent BCM
  • Failed or intermittent control module on the same bus
  • Missing or incorrect termination resistor(s)
  • Low battery or unstable supply voltage

Symptoms

  • Multiple body systems inoperative or intermittent (locks, lighting, windows, HVAC controls)
  • One or more control modules not present on scan tool
  • Intermittent operation or random faults that clear and return
  • Warning lights or DIC messages related to communications
  • Possible no-crank/no-start if startup messages are lost (vehicle dependent)

What to check

  • Read all modules with a capable scan tool; note which modules fail to respond and any network-related U-codes
  • Check vehicle battery voltage and BCM supply/ground circuits at rest and during cranking
  • Visually inspect connectors and wiring for corrosion, bent pins, damage and water ingress
  • Confirm termination resistance across CAN/LIN pair with ignition off (typical CAN ≈ 60 Ω)
  • Perform wiggle test while monitoring network with a scan tool or scope to reproduce fault
  • Disconnect suspected aftermarket devices and repeat scan

Signal parameters

  • Typical CAN recessive voltage: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (both equal)
  • Typical CAN dominant voltages: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ≈ 2.0 V)
  • Bus idle differential ≈ 0 V (small), dominant differential ≈ 2.0 V
  • Termination resistance (two 120 Ω in parallel) ≈ 60 Ω across CAN_H and CAN_L with ignition OFF
  • Common data rates: 125 kb/s, 250 kb/s, 500 kb/s (verify vehicle-specific bus speed)

Diagnostic algorithm

  1. Connect a professional scan tool and record all stored/pending codes and module presence. Note any U-codes or additional B-codes.
  2. Check battery voltage and BCM power/ground circuits. Repair any low or intermittent supply first.
  3. Visually inspect BCM connectors and wiring along Data Bus B for damage, corrosion or water entry. Repair or clean as needed.
  4. With ignition off, measure resistance across the bus (CAN_H to CAN_L) at the BCM connector to confirm ~60 Ω. If open or shorted, isolate wiring or connectors.
  5. With key on, measure CAN_H and CAN_L voltages at the BCM connector. Verify recessive and dominant levels with a scope while exercising the bus. Look for missing pulses, stuck dominant state, or abnormal noise.
  6. If a short-to-power or short-to-ground is suspected, disconnect modules on Bus B one at a time (or remove suspected connectors) and re-check the bus to locate the offending node.
  7. Perform a wiggle test on harnesses while monitoring communication to reproduce intermittent faults.
  8. If wiring and other modules check good, attempt reprogramming or reflashing BCM software per manufacturer procedures before replacement (if applicable).
  9. Replace the faulty module only after confirming wiring and bus integrity. Clear codes and verify repair with a road test and re-scan.

Likely causes

  • Loose or corroded connector at the BCM or a module on Bus B
  • Damaged wiring harness pinched or chafed causing short/open
  • Failed termination resistor or incorrect termination resistance
  • One module pulling the bus to dominant state (short to battery/ground)
  • Faulty BCM or other module causing bus errors

Fault status

⚠️ Status
BCM detected interrupted or invalid communication on Data Bus 'B'. Codes set when expected messages time out or the bus is held in a dominant state. May be intermittent or permanent depending on wiring, module or termination faults.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1-3 hours

Similar codes

U0073 U0077 U0100 U0140 B2400 B2401
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