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Home / DTC / B3901 — Body Control Module — Internal Software Error

B3901 — Body Control Module — Internal Software Error

Detailed page for trouble code B3901.

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Code

B3901

Generic B — Body

Body Control Module — Internal Software Error

Brand: Generic
Type: B — Body
Views: UK: 0 EN: 2 RU: 0
AI status
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Page language: EN

Causes

  • Corrupted or incomplete software/flash image
  • Interrupted or failed module reprogramming
  • Low battery or unstable power during programming
  • Internal memory (flash/NVRAM) or processor fault
  • Software calibration mismatch after module swap
  • Loss of power or supply/ground faults to BCM

Symptoms

  • BCM not responding to scan tool or limited/partial response
  • Erratic or nonfunctional body functions (lights, locks, windows, wipers)
  • Multiple body/system DTCs or communication faults present
  • Instrument cluster or dash warnings related to BCM or communications
  • Stored U‑codes (CAN/comm) alongside B3901
  • Vehicle may not retain settings or has unusual behavior after power cycle

What to check

  • Read freeze frame and full DTC list from BCM and other modules; note timestamps and related U/P codes
  • Verify battery state of charge and system voltage (preferably 12.4–12.8 V or manufacturer spec)
  • Inspect BCM connectors, pins, and harness for corrosion, water, or damage
  • Check BCM power and ground circuits for proper voltage and continuity
  • Scan CAN/LIN bus for bus‑off, error counters, or other modules losing communication
  • Attempt to read BCM part number, software/calibration IDs and boot status via OEM scan tool

Signal parameters

  • Battery voltage at BCM (key on and during cranking)
  • BCM supply and ground continuity/resistance
  • CAN High/Low bus activity and error counters
  • BCM module response codes, software/calibration part numbers, boot status/uptime
  • Number and type of stored fault codes (BCM and related modules)
  • Programming status flags or KAM (keep alive memory) errors if available

Diagnostic algorithm

  1. Connect a capable OEM or professional scan tool and retrieve all stored and pending codes from BCM and other modules; save reports
  2. Record freeze frame data and note vehicle voltage during events
  3. Verify battery charge and health; if low, charge and retest (do not program if battery is low)
  4. Visually inspect BCM enclosure and connector for corrosion, water ingress, or physical damage; repair as needed
  5. Verify BCM power and ground circuits (voltage with key on, engine off, and continuity to chassis ground)
  6. Check CAN bus integrity: measure CAN differential voltages, verify termination resistors and that other modules can communicate
  7. Clear codes and perform a controlled power cycle; see if B3901 returns and whether it is intermittent
  8. If hardware and bus are good and software/cal IDs are accessible, check OEM for software updates/TSBs or reprogramming procedures
  9. Attempt to reflash/reprogram the BCM with the correct factory calibration using OEM tool and stable power source (follow OEM precautions — battery charger, maintained voltage)
  10. If reprogramming fails or module will not accept correct software, consider BCM replacement and ensure correct pairing/immobilizer coding if required
  11. After repair/reprogramming, clear codes, reinitialize vehicle settings, and perform full systems communication check and a road test to verify functions remain stable

Likely causes

  • Recent BCM reflash or dealership/aftermarket programming interrupted
  • Low battery voltage during a previous update or battery replacement
  • Failed internal flash memory or worn NVRAM cells
  • Loose/poor power or ground connection to BCM
  • Incompatible software loaded to the module
  • Repeated CAN bus errors or bus-off condition preventing normal initialization

Fault status

⚠️ Status
BCM detected internal software/flash error or failed self‑test; module may be unresponsive or running in fallback mode. Reprogramming or module replacement likely required after verifying power, grounds and CAN network.
🔴 Repair difficulty: Hard
⏱️ Diagnostic time: 2.0-4.0 hours

Similar codes

B3900 B3902 B3910 U0073 U0100 U0401
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