B257B

Code

Generic B — Body

Body Control Module Internal Fault — Unexpected Reset

Brand: Generic

Type: B — Body

Views: UK: 7 EN: 14 RU: 13

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Intermittent or low battery/charging system voltage
Loose, corroded, or damaged power/ground connections to the BCM
Voltage transients or spikes (load dump, jump start issues)
Internal BCM hardware failure (processor, memory, watchdog)
Corrosion, water ingress, or physical damage to the module or connector
Faulty or corrupted BCM software/firmware

Symptoms

Stored B257B code and possibly other related body/network codes
Intermittent loss of BCM-controlled functions (door locks, lighting, windows)
Instrument cluster or infotainment reboots or communication loss
Unexpected module resets noted in diagnostic logs or freeze frame
Fuses appear intact but circuits behave unpredictably
Possible illumination of warning lights or message about module malfunction

What to check

Read all stored codes and freeze frame data from BCM and related modules; save logs
Check battery state of charge and charging system voltage at rest and while cranking
Inspect BCM connector for corrosion, bent pins, signs of water ingress, or damage
Verify power and ground pin voltages at the BCM connector with key on (see signal_params)
Scan network traffic on CAN/LIN for repeated errors, bus-off events, or timeouts
Confirm no recent aftermarket installations (alarms, remote start) are connected to BCM circuits

Signal parameters

VBATT at BCM connector: ~12.4–12.8 V (key off), ~13.8–14.6 V (engine running)
Switched 12V (IGN) at BCM when key on: near battery voltage
Ground voltage:
CAN high/CAN low idle voltages: ~2.5 V each (differential ~0 V), no bus-off errors
LIN line (if present): idle voltage per manufacturer spec (typically Vbat or pull-up level)
Module internal temperature (if available): within normal operating range (no thermal runaway)

Diagnostic algorithm

Capture the BCM diagnostic report, freeze-frame, and any crash/reset counters. Note time and conditions of last reset.
Verify battery condition and charging system: measure resting voltage, apply load test, measure alternator output with engine running.
Visually inspect BCM connector and harness for corrosion, water intrusion, pin damage, or poor mating. Remove connector and inspect contacts if safe.
With connector connected, backprobe power and ground pins while monitoring voltage during key-on, cranking, and engine run to check for drops or transients.
Check vehicle grounds related to BCM (chassis and engine grounds). Tighten and clean ground points; re-measure ground voltage under load.
Scan the vehicle bus for CAN/LIN errors, repeated node restarts, or bus-off status. Identify if other modules report communication errors around the BCM reset time.
If aftermarket accessories are present, disconnect them and retest to rule out interference or inadequate power wiring.
If hardware and wiring check OK, check for available BCM software/firmware updates and manufacturer service bulletins. Reflash or reprogram BCM per factory procedure if an update or corruption is suspected.
If problem persists after wiring and software actions, consider swapping in a known-good, VIN‑programmed BCM (if available) or bench-testing the module per factory procedures. Reprogram and code replacement module as required.
After repairs, clear codes, reproduce the conditions that previously caused the reset, and road test while monitoring for recurrence. Record findings and final status.

Likely causes

Poor battery or charging system voltage (most common)
Faulty/weak ground or power connector at the BCM
Corroded/wet connector causing intermittent supply
Software/firmware bug or corrupted calibration requiring reflash

Fault status

⚠️ Status

BCM internal fault detected — unexpected module reset/restart recorded. Investigate power, ground, connector integrity, CAN/LIN communications, and software integrity.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.5-3.5 hours