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 U

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 U indicates an immobilizer authentication failure between the Body Control Module and key reader coil, preventing startup or high-voltage power engagement. This high-severity issue is typically caused by a faulty immobilizer coil 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 U
  • 1Internal open circuit in the Immobilizer Coil, or a loose or oxidized connector, causing a failure to read the key chip signal.
  • 2BCM software version defect (e.g., authentication logic flaws in early versions such as Ver 1.02) or poor solder joints on the internal EEPROM chip.
  • 3Low smart key battery (<2.8V) or damaged key chip, resulting in insufficient signal strength.
  • 4Interior/exterior low-frequency (LF) antenna fault, backed-out connector terminal, or wiring harness short to ground, causing key positioning failure.
  • 5CAN bus communication fault (abnormal CAN-H/CAN-L voltage) or poor contact in 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 along with any accompanying U-class 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 fault with the key itself.
  • 3
    Measure the immobilizer reader coil resistance (standard value 0.9-1.5 Ω). Check the coil connector for looseness, oxidation, or backed-out pins. 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 (G301, etc.), and CAN line voltage (CAN-H 2.6V / CAN-L 2.4V, terminal resistance 60Ω).
  • 5
    Perform the low-frequency antenna actuation test. Check if the front interior, rear interior, and exterior low-frequency antennas operate normally, and 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 solder joints of the anti-theft authentication chip inside the BCM, 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 U Fault Codes
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.B110AThis DTC indicates interrupted CAN bus communication or abnormal data between the air conditioning control unit (ACU) and the PM2.5 air quality sensor. The PM2.5 sensor monitors particulate concentration inside and outside the vehicle and provides the basis for purification control in the automatic air conditioning system. The ACU triggers this fault if it fails to receive a valid CAN message from the sensor within the specified period (usually 100-500 ms), if message verification fails, or if the signal frame times out. Although this fault does not affect vehicle driving safety, it disables the automatic air purification function. The air conditioning system cannot automatically switch between recirculation and fresh air modes or activate the air purification mode based on air quality. A persistent fault may restrict the thermal management strategy.B110A02DTC B110A02 indicates the thermal management master control unit (HVAC ECU) or body control module (BCM) failed to receive valid CAN data frames from the PM2.5 air quality sensor over consecutive communication cycles (typically 500ms per cycle). The sensor typically mounts at the HVAC unit air inlet or behind the glove box and transmits real-time particulate concentration data (unit: μg/m³) to the HVAC controller via the CAN bus. The fault is essentially a communication link layer timeout. Potential causes include physical layer wiring faults, sensor power supply failures, sensor internal MCU lockup, or CAN network topology damage. Although this non-critical fault does not directly affect driving safety, it disables the intelligent air purification strategy and the automatic fresh air/recirculation switching function. In extreme cases, it triggers a thermal management system degraded mode, limiting compressor power or fixing the air flap position.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.B173712DTC B173712 indicates a short to power in the firing circuit of the right second-row side airbag module in the Supplemental Restraint System (SRS). Specifically, the "right second-row side airbag" typically refers to the side impact airbag located in the right B-pillar or the side of the right seat. The "12" suffix indicates a circuit short to battery positive (B+). This fault causes the following conditions: 1) The airbag may fail to ignite and deploy during a collision, resulting in a loss of side impact protection. 2) In extreme cases, unintended triggering may cause accidental airbag deployment. 3) The SRS control unit detects abnormally high circuit voltage (typically above 4.5V) and enters fault protection mode. This may disable the entire airbag system and continuously illuminate the instrument cluster airbag warning light.
Common problems reported for Seal U →Seal U 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.