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B1621-00

DTC B1621-00 indicates a short to ground in the driver side airbag (typically the seat-mounted side airbag or side curtain airbag) igniter circuit — Atto 8

Safety System

DTC B1621-00 indicates a short to ground in the driver side airbag (typically the seat-mounted side airbag or side curtain airbag) igniter circuit.

In the BYD SRS system, the ECU continuously monitors the circuit resistance of each airbag igniter (normal value: approximately 2.0-5.0 Ω).

When the ECU detects resistance below 1.0 Ω or voltage close to 0 V, it identifies a short to ground.

This fault causes the system to disable the affected airbag circuit, preventing normal deployment during a side impact.

In extreme cases, unstable resistance at the short circuit causes intermittent contact, posing a risk of unintended airbag deployment.

When this DTC triggers, the SRS control unit illuminates the instrument cluster airbag warning light and stores freeze frame data recording the vehicle status at the time of the fault.

5
Cases Logged
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Worn seat rails caused a wiring harness short circuit.

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Model: 2017 Qin EV300, 82,000 km. Symptom: Airbag warning light continuously illuminated on the instrument panel. DTC: B1621-00 (current fault). Diagnosis: Inspected the wiring harness beneath the seat and found the protective sleeve worn at the left rail of the driver's seat, with internal copper wires exposed and contacting the seat frame. Long-term friction from frequent seat adjustments wore through the insulation. Repair: Repaired the damaged harness using heat-shrink tubing for insulation, rerouted the wiring and added protective corrugated tubing, then secured the harness to non-moving sections of the seat frame using cable ties to prevent interference with the seat rails. Cleared the fault code and the system returned to normal.
BYD DTC AI AnalysisFrom Chinese market (translated)

Connector water ingress and corrosion after wading

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Model: 2018 BYD Qin EV450. Driven through floodwater after heavy rain. Fault symptom: After drying, the airbag warning light illuminated upon starting, logging DTC B1621-00. Diagnosis: Inspected the SRS connector under the driver's seat. Found muddy water residue inside the yellow connector plug, with green copper corrosion on the terminal surfaces creating a short circuit path between terminals and the connector housing. Resolution: Disconnected the battery. Thoroughly cleaned the connector terminals with electronic contact cleaner. Dried them with compressed air, then applied DENSO conductive paste (or equivalent protectant). Replaced the waterproof seal ring and ensured the connector locking tab was fully seated. Cleared fault codes and tested; fault resolved. Recommend checking the vehicle floor seals to prevent further water ingress.
BYD DTC AI AnalysisFrom Chinese market (translated)

Misdiagnosed as spiral cable internal short circuit

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Vehicle: 2017 Qin 100, collision-repaired. Symptom: B1621-00 DTC appeared after replacing front bumper and driver seat. Diagnosis: Initially suspected the replacement seat side airbag, but the fault persisted after substituting it with a simulator. Further inspection found that steering wheel removal had damaged the clock spring (spiral cable) tail harness, causing the side airbag circuit wires to short against the steering column. Repair: Replaced clock spring assembly (part number must match VIN), recalibrated steering angle sensor. Fault cleared. Tip: Some Qin models route side airbag circuit signals through the clock spring. Take care during repairs.
BYD DTC AI AnalysisFrom Chinese market (translated)

Airbag module internal igniter short circuit

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Vehicle: 2017 BYD Qin 80, involved in a side collision. Symptom: After accident repairs (damaged door and airbag replaced), the airbag warning light came on with DTC B1621-00. Diagnosis: Replaced the airbag module with a simulator, and the fault cleared. Measured the old module's resistance: 0.3Ω (well below normal 2–5Ω). This confirmed a short circuit in the igniter bridge wire inside the airbag inflator. The short circuit likely resulted from igniter deformation caused by the impact, or a parts quality issue. Fix: Replaced the driver's side airbag module with a genuine unit, verifying the part number matched the vehicle configuration since high-spec and base models use different airbags. Verified the connector shorting bar worked properly before installation. Configured the SRS system after installation. Fault resolved.
BYD DTC AI AnalysisFrom Chinese market (translated)

SRS control unit internal circuit fault

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Vehicle: 2017 BYD Qin EV300. The fault kept coming back after several repair attempts. Symptoms: The car repeatedly stored DTC B1621-00. The wiring harness and airbag module had been replaced earlier, but the fault stayed intermittent — especially over bumps. Diagnosis: Harness resistance tested within spec. Vibration testing revealed an intermittent short to ground at the control unit connector. We removed and inspected the SRS control unit (under the centre console or dash) and found cold solder joints on the internal circuit board, causing a fault in the driver's side airbag deployment circuit. Repair: We replaced the SRS control unit, programmed it to match the vehicle (writing the VIN and configuration codes), then ran system calibration and crash sensor zero-point calibration. The fault never came back.
Other Atto 8 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.