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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.

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Cases Logged
Real-World Cases
BYD DTC AI Analysis

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

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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.
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BYD DTC AI Analysis

F3 immobiliser coil open circuit causes no start

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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.
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BYD DTC AI Analysis

BCM internal fault prevented Qin Plus EV from powering on

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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.
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BYD DTC AI Analysis

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

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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.
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Other Seal U 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.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.
Data confidence: Community This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself.