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) — Qin Plus

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 Qin Plus 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. This high-severity fault is commonly caused by a faulty immobilizer coil or BCM software defects. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available from 4 recorded cases.
4
Cases Logged
5
Causes
Likely Causes — Qin Plus
  • 1Internal open circuit in the immobilizer coil or a loose/oxidized connector, preventing the system from reading the key chip signal.
  • 2BCM software defect (early versions such as Ver 1.02 contain authentication logic flaws) or poor solder connections 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 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, terminating resistance 60Ω).
  • 5
    Perform the low-frequency antenna operation test. Check the front interior, rear interior, and exterior low-frequency antennas for normal operation. Measure the antenna resistance (standard: 1.5-2.0kΩ).
  • 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 solder joints of the immobilizer authentication chip inside the BCM, or replace the BCM, perform online programming, and perform all-keys-lost matching (8-digit PIN required).
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 Qin Plus 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.B110913DTC B110913 indicates an open signal circuit for the PM2.5 rapid detector (in-cabin air quality sensor). This sensor typically mounts inside the air conditioning system intake duct. It monitors in-cabin PM2.5 concentration in real time and provides data to the automatic air conditioning system for intelligent control of the air purification function. The BYD diagnostic protocol uses sub-code '13' to specifically indicate an open signal circuit. Although this fault theoretically affects a comfort feature, BYD Qin series vehicles classify it as a Level 3 (severe) fault. This classification occurs because the air conditioning system interacts with the battery thermal management system (e.g., monitoring battery cooling intake air quality). The fault can disable the automatic air conditioning mode and prevent the air purification function from operating. In extreme cases, it affects the battery compartment intake air quality assessment, triggering the 'repair immediately' strategy.B110BDTC B110B indicates a fault in the internal air pump (sampling pump) of the PM2.5 air quality detector. This pump draws cabin or outside air into the PM2.5 sensor detection chamber, enabling the laser scattering sensor to measure real-time particulate matter concentration. The air conditioning controller (or BCM) sets this DTC upon detecting an open or short circuit in the pump drive circuit, or abnormal current caused by a seized pump motor. This fault disables the "Green Clean" system, causes the instrument cluster air quality display to read abnormally or disappear, and disables the automatic fresh air/recirculation switching function. It does not affect the vehicle powertrain or core thermal management functions.B173A1ADTC B173A1A indicates the right middle-row side airbag (typically located on the side of the right second-row seat or inside the C-pillar trim panel) igniter circuit resistance measures 0 Ω. In the Supplemental Restraint System (SRS), normal airbag igniter resistance ranges between 2.0 and 5.0 Ω. A resistance of 0 Ω indicates a short to ground or a short between circuit wires, preventing the Airbag Control Unit (ACU) from establishing a normal monitoring circuit. This safety-critical fault prevents the airbag from deploying during a collision, continuously illuminates the instrument panel SRS warning lamp, and may force the entire airbag system into fail-safe mode.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 Qin Plus →Qin Plus 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.