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 — Seal U

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 — Seal U
  • 1Under-seat wiring harness wear: Frequent fore-aft movement of the driver's seat wears through the side airbag wiring harness insulation (usually located on the inner side of the seat rail), causing it to contact the metal vehicle frame and create a short to ground.
  • 2Connector water ingress and corrosion: Water enters the yellow SRS connector under the seat due to vehicle wading, a blocked sunroof drain tube, or car washing, causing a short circuit between terminals or between a terminal and the housing.
  • 3Internal short circuit in the spiral cable (clock spring): The internal flat ribbon conductor in the spiral cable beneath the combination switch breaks and shorts to ground, or harness chafing 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 ignition driver transistor inside the control unit 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 a dedicated BYD 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.). Check for accompanying fault codes (such as B1622 passenger side short circuit).
  • 3
    Visual inspection: Inspect the yellow SRS connector under the driver's seat (usually located on the left side of or beneath the seat rail) for physical damage, signs of water ingress, corrosion, or looseness; inspect the wiring harness for signs of pinching or wear from the seat rail.
  • 4
    Harness continuity test: Disconnect the battery negative terminal and unplug the SRS control unit connector. Use a digital multimeter to measure the resistance between the pin corresponding to B1621 (refer to the wiring 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 side airbag connector (located under the seat, usually protected by a shorting bar). Install an airbag simulator (dedicated 2Ω resistor) in place of the airbag module. Reconnect the battery, clear the fault codes, and read the data stream. Check if the resistance value returns to normal. If normal, the airbag module is faulty. If the short circuit persists, the fault lies in the wiring harness or control unit.
  • 6
    Spiral cable inspection (if applicable): For models where the circuit passes through the clock spring, disconnect both ends of the spiral cable. Measure the resistance to ground for the airbag-side harness and the control unit-side harness to inspect the steering column wiring harness.
  • 7
    Repair and replacement: If the wiring harness is damaged, repair the insulation using heat-shrink tubing or electrical tape, and re-secure the harness to prevent further chafing; if the connector has water ingress, clean it with electrical contact cleaner and apply conductive grease, or 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 using 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 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: 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]