AU
B1681-00

On BYD vehicles, B1681-00 typically indicates an immobilizer authentication failure or communication fault between the Body Control Module (BCM), the smart key, and the immobilizer reader coil (antenna) — Seal 6 EV

Safety System

On BYD vehicles, B1681-00 typically indicates an immobilizer authentication failure or communication fault between the Body Control Module (BCM), the smart key, and the immobilizer reader coil (antenna).

This fault prevents the BCM from verifying key validity, triggering the anti-theft lockout.

Symptoms include failure to start, failure to engage high-voltage power (OK mode), or the instrument cluster displaying "Please check the starting system".

Although some documents describe this code as an airbag switch short to ground, repair experience and fault code definitions confirm it primarily points to an interrupted immobilizer authentication process involving the low-frequency antenna, immobilizer coil, or a BCM software or hardware fault.

The B1681-00 fault code on the Seal 6 EV indicates an immobilizer authentication failure between the Body Control Module, smart key, and reader coil, preventing the vehicle from starting or engaging high-voltage power due to anti-theft lockout. Severity is high. Common causes include a faulty immobilizer coil, loose connector, or BCM software defect. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available based on 4 recorded cases.
4
Cases Logged
5
Causes
Likely Causes — Seal 6 EV
  • 1Internal open circuit in the immobilizer coil, or a loose or oxidized connector, preventing the system from reading the key chip signal.
  • 2BCM software version defect (e.g., authentication logic flaws in early versions such as Ver 1.02) or poor solder joint on the internal EEPROM chip.
  • 3Low smart key battery (<2.8V) or damaged key chip causing insufficient signal strength.
  • 4Interior/exterior low-frequency (LF) antenna fault, backed-out connector terminal, or wiring harness short to ground, causing key localization failure.
  • 5CAN bus communication fault (abnormal CAN-H/CAN-L voltage) or poor contact in the BCM power supply/ground circuit.
What To Do
  • 1
    Connect the VDS2000 or Launch X431 diagnostic tool, read all BCM (Body Control Module) fault codes, and confirm B1681-00 and any accompanying U-series communication or antenna fault codes.
  • 2
    Check the smart key battery voltage (standard ≥3.0V) and signal strength. Test with the spare key to rule out a faulty key.
  • 3
    Measure the immobiliser reader coil resistance (standard value: 0.9-1.5 Ω). Check the coil connector for looseness, oxidation, or backed-out terminals. Clean the connector or replace the ignition lock housing assembly if necessary.
  • 4
    Check the BCM power supply fuse (e.g., F1/23 15A), ground points (e.g., G301), and CAN line voltage (CAN-H 2.6V / CAN-L 2.4V, terminal resistance 60Ω).
  • 5
    Perform the low-frequency antenna operation test. Verify normal operation of the front interior, rear interior, and exterior low-frequency antennas. Measure the antenna resistance (standard: 1.5-2.0 kΩ).
  • 6
    Check the BCM software version. If the version is defective (such as Ver 1.02), update to the latest version (Ver 1.05+) and perform immobilizer system matching.
  • 7
    If all above checks are normal, remove and inspect the internal BCM anti-theft authentication chip solder joints, or replace the BCM, perform online programming, and complete all-keys-lost matching (requires 8-digit PIN code).
Real-World Cases
BYD DTC AI Analysis

BYD Song Pro DM-i no-start due to software version defect

✓
2022 Song Pro DM-i failed to start after parking. The dashboard displayed ‘Check Start System’ and the immobiliser indicator flashed continuously. A diagnostic scan retrieved DTC B1681-00 stored in the BCM. Immobiliser coil resistance measured 1.2Ω (normal), and key voltage read 3.0V (normal). The BCM was running software Ver 1.02, which contains an immobiliser authentication logic flaw. Updated BCM software to Ver 1.05+ and performed immobiliser system matching to resolve the fault.
Original source ↗
BYD DTC AI Analysis

F3 immobiliser coil open circuit causes no start

✓
2015 F3 stalled while driving and failed to restart. The immobiliser indicator flashed rapidly. Retrieved DTC B1681-00 (transponder coil communication fault) from the immobiliser control system. Checked and found the ignition lock barrel transponder coil connector loose with oxidised pins. Measured infinite resistance across the coil (specification 1.0–1.5 ohms), indicating an internal open circuit. Replaced the ignition lock barrel assembly (F3-3704010), re-matched both keys, and the vehicle started normally.
Original source ↗
BYD DTC AI Analysis

BCM internal fault prevented Qin Plus EV from powering on

✓
2021 Qin Plus EV: Pressing the brake pedal failed to engage OK mode; the instrument panel displayed 'Check Power System'. VDS scan showed BCM code B1681-00 (BCM to immobilizer module communication fault) and U-code communication faults. Checked BCM power supply, ground, and CAN lines — all normal. Disconnected and reconnected the battery; the fault persisted. Removed and inspected the BCM, finding cold solder joints on the internal immobilizer authentication chip (EEPROM). Replaced the BCM, performed online programming and all-keys-lost matching. Fault eliminated.
Original source ↗
BYD DTC AI Analysis

Tang DM low-frequency antenna fault causing intermittent no-start

✓
2018 Tang DM had an intermittent no-start condition, more likely after exposure to high temperature and direct sunlight. The instrument cluster displayed 'key not detected', but emergency start worked normally. Read DTCs B1681-00 (low-frequency antenna fault) and B1682. Performed a low-frequency antenna actuation test: the front interior low-frequency antenna (behind the start button on the centre console) did not respond. Inspected the connector and found a pin had backed out. Replaced the front interior low-frequency antenna (6A-3774010), updated the IKCU software, and cleaned all antenna connectors. The fault has not returned.
Original source ↗
Other Seal 6 EV Fault Codes
B1108For the BYD Song MAX (2017–2019), DTC B1108 indicates a short circuit in the PM2.5 air quality detection module (dust sensor) or its wiring harness. The sensor uses laser scattering to detect in-cabin and outside PM2.5 concentrations and transmits data to the HVAC ECU via the LIN bus or an analog signal. Short circuit conditions include: 1) Sensor power supply wire (+5V/+12V) shorted to ground 2) Signal wire shorted to power or ground 3) Short circuit in the sensor's internal photoelectric detection circuit This fault prevents the automatic climate control from switching between fresh air and recirculation based on air quality. The system enters fail-safe mode (typically forced recirculation). Severe short-circuit current can destroy the HVAC ECU sampling circuit. The system classifies this as a Level 3 severe fault and restricts vehicle operation to prevent electrical fires or controller damage. For other BYD models (such as the Qin EV, E5, and Song DM), B1108 indicates a driver-side sunload sensor fault, reflecting differences in DTC definitions across models.B110811This DTC indicates a short circuit in the air conditioning system PM2.5 air quality sensor (rapid detector). Specifically, abnormal continuity exists between the sensor power supply circuit or signal output circuit and body ground or battery positive. This causes the control module (ACU/air conditioning controller) to detect abnormal current or a voltage signal outside the valid range (typically 0V or supply voltage). The PM2.5 sensor monitors internal and external particulate concentrations, providing the data the automatic air conditioning system uses to switch between fresh air and recirculation modes. A short circuit disables the air quality monitoring function. This fault may force the air conditioning system into recirculation protection mode and trigger related thermal management system faults, as the air conditioning system acts as a key actuator in vehicle thermal management. In severe cases, the short circuit current can damage the internal sampling circuit of the air conditioning controller or overheat the wiring harness.B1109DTC B1109 indicates an open circuit in the communication line between the air conditioning system PM2.5 air quality sensor (dust concentration sensor) and the air conditioning control unit (AC ECU), or an open circuit within the sensor internal circuitry. Typically located in the air conditioning intake duct, this sensor monitors real-time PM2.5 particle concentration inside and outside the vehicle. It provides data to the automatic air conditioning system for intelligent switching between fresh air and recirculation modes and for controlling the air purification device. During an open circuit fault, the AC ECU cannot receive the air quality signal, triggering the system fail-safe mode. Symptoms typically include failure of the automatic recirculation function, abnormal air purification indicator light operation, or abnormal air quality data displayed on the air conditioning panel. Severe cases may compromise the cabin environment control strategy, but do not directly affect vehicle driving safety.B1737DTC B1737 indicates the Supplemental Restraint System (SRS) detected abnormal continuity between the right middle-row side airbag squib circuit (typically the second-row right seat side airbag or right curtain airbag) and the vehicle power supply (B+, battery positive). The SRS ECU monitors the airbag inflator resistance via an internal current source (normal resistance: 2.0–3.0 Ω). If the ECU detects the voltage across the inflator circuit remaining close to battery voltage (>5 V and approaching 12 V) instead of the expected low potential or floating state, it identifies a short to power. This fault prevents the airbag from deploying normally during a collision because the short circuit bypasses the firing current. In extreme cases, abnormal current can cause unintended deployment, posing a severe safety hazard.B173BDTC B173B indicates an abnormal electric A/C compressor speed feedback signal or a communication fault between the compressor controller and the motor. In BYD new energy models (e.g., Tang, Song, Qin, and Yuan series), this DTC belongs to the HVAC subsystem within the Body Control system and specifically involves the electric compressor closed-loop control circuit. The ECU sets this DTC when the compressor controller (IPM) fails to receive a valid pulse signal from the Hall speed sensor for more than 3 continuous seconds, or when the deviation between the detected actual speed and the target commanded speed exceeds the calibrated threshold (typically >500 rpm). This fault forces the A/C system into fail-safe mode and stops compressor operation. Symptoms include a complete lack of cooling, intermittent cooling, or a sharp decrease in cooling performance. The instrument cluster A/C warning lamp may also illuminate. Prolonged operation with this fault can degrade high-voltage system insulation or cause controller overcurrent damage.
Common problems reported for Seal 6 EV →Seal 6 EV overview →
Data confidence: Official This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself.