AU
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 3

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
5
Causes
Likely Causes — Atto 3
  • 1Worn under-seat wiring harness: Frequent fore/aft driver seat movement wears through the insulation of the side airbag wiring harness (usually located on the inner side of the seat rail). The exposed wire contacts the vehicle metal frame, creating a short to ground.
  • 2Connector water ingress and corrosion: Water enters the yellow SRS connector under the seat during vehicle wading, car washing, or due to a blocked sunroof drain tube, causing a short circuit between terminals or between a terminal and the housing.
  • 3Internal short circuit in the spiral cable (clock spring): The flat ribbon wire inside the spiral cable beneath the combination switch breaks and shorts to ground, or a chafed wiring harness near the steering column causes a short circuit (on some models, the side airbag circuit passes through this area).
  • 4Airbag module internal fault: Short circuit in the igniter bridge wire inside the airbag inflator, or deformed igniter socket terminal contacting the housing.
  • 5SRS control unit internal driver circuit fault: The internal ignition driver transistor breaks down, causing a false detection or an actual short to ground (less common).
What To Do
  • 1
    Safety preparation: Shift the vehicle into P or Neutral, apply the handbrake, disconnect the 12V battery negative terminal, and wait at least 90 seconds to fully discharge the SRS capacitor and prevent accidental airbag deployment.
  • 2
    Fault Confirmation and Freeze Frame Analysis: Use the BYD dedicated diagnostic tool (BYD-EDS or Launch X431) to read the fault code. Confirm B1621-00 is a current fault (Active). Record freeze frame data (mileage, temperature, voltage, etc.) and check for accompanying fault codes (e.g., B1622 passenger side short circuit).
  • 3
    Visual inspection: Check the yellow SRS connector under the driver’s seat (usually on the left side of or beneath the seat rail) for physical damage, signs of water ingress, corrosion, or looseness. Check the wiring harness for pinch marks or chafing from the seat rail.
  • 4
    Harness continuity test: Disconnect the battery negative terminal, unplug the SRS control unit connector, and use a digital multimeter to measure the resistance between the pin corresponding to B1621 (refer to the circuit diagram, usually the driver-side airbag circuit low-side pin) and body ground. Normal resistance is infinite (OL). Continuity indicates a short circuit in the harness.
  • 5
    Airbag module isolation test: Disconnect the driver-side airbag connector (located under the seat, usually protected by a shorting bar). Install an airbag simulator (dedicated 2Ω resistor) to replace the airbag module. Reconnect the battery, clear the fault codes, and read the data stream to check if the resistance value returns to normal. If normal, the airbag module is faulty. If the short circuit persists, the wiring harness or control unit is faulty.
  • 6
    Spiral cable check (if applicable): For models where the circuit passes through the clock spring, disconnect both ends of the spiral cable. Measure the resistance to ground of the airbag-side wiring harness and the control-unit-side wiring harness respectively to inspect the wiring harness in the steering column area.
  • 7
    Repair and Replacement: If the wiring harness shows damage, repair the insulation using heat-shrink tubing or electrical tape, and re-secure the harness to prevent further chafing. If water has entered the connector, clean it with electrical contact cleaner, apply conductive grease, and replace the connector if necessary. If the airbag module or control unit is faulty, replace it with a new part.
  • 8
    System verification: After repair, clear the fault code and cycle the ignition switch (ON-OFF-ON) to confirm the fault code does not return. Perform an SRS system self-check (some models require a diagnostic tool to execute the 'Configuration and Settings' function). Finally, perform a road test to confirm the airbag warning light turns off normally.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Worn seat rails caused a wiring harness short circuit.

✓
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

✓
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

✓
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

✓
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

✓
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 3 Fault Codes
P0A0DBattery Energy Control Module Requested MIL IlluminationP0AA6Hybrid Battery Voltage System Isolation FaultP1A00Drive Motor "A" Performance — BYD proprietaryB1108For 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.
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. Sources: [1]