AU
B1620-00

DTC B1620-00 indicates the airbag control unit (SRS ECU) detects an open circuit or disconnected electrical connection at the left front seat side airbag (Driver Side Airbag) — Seal U

Safety System

DTC B1620-00 indicates the airbag control unit (SRS ECU) detects an open circuit or disconnected electrical connection at the left front seat side airbag (Driver Side Airbag).

Specifically, this fault indicates a circuit interruption between the seat-integrated side airbag module (typically located on the side of the seat backrest) and the vehicle wiring harness.

The SRS ECU continuously monitors the airbag igniter circuit resistance (standard value: 2.0-3.0Ω) via a low-level signal line.

The ECU sets this DTC when the resistance exceeds the threshold (typically >6Ω or open circuit) for longer than the specified time (typically >2 seconds).

This is a hard fault that prevents the left front seat side airbag from deploying during a collision.

The system also illuminates the instrument cluster SRS warning lamp and may disable the entire airbag system deployment function, severely compromising passive safety.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Loose or disconnected yellow airbag connector under the seat: Sliding the seat forward and backward or adjusting the seat height can loosen this dedicated yellow connector (typically equipped with a short-circuit protection tab). This is the most common root cause of this fault code.
  • 2Seat side airbag wiring harness wear and breakage: Frequent seat adjustment repeatedly bends the wiring harness at the slide rail mounting point, causing fatigue fracture of the internal copper core. The outer insulation remains visibly intact, creating a hidden open circuit.
  • 3Side airbag igniter internal open circuit: The igniter bridge wire inside the airbag module blew or has poor contact. Airbag aging or incorrect replacement after a previous accident typically causes this.
  • 4Corrosion of the wiring harness connector between the SRS ECU and the seat: Liquid intrusion during vehicle wading or interior cleaning oxidizes the connector terminals under the seat or lower B-pillar, increasing contact resistance.
  • 5Clock spring or slip ring assembly fault: On some models, the seat side airbag wiring harness routes through the rotary connector under the seat. Long-term wear causes an internal open circuit.
What To Do
  • 1
    Safety preparation and fault confirmation: Disconnect the 12V battery negative terminal and wait at least 3 minutes. Use the dedicated diagnostic tool to read and confirm B1620-00 as a current fault (Active), record the freeze frame data, and check for accompanying fault codes (such as B1621 right front seat side airbag).
  • 2
    Visually inspect the connector under the seat: Move the seat to the fully forward and fully rearward positions. Verify the yellow dedicated connector under the seat (usually marked 'AIRBAG' or 'SRS') is fully locked. Inspect the shorting bar inside the connector for normal condition. Disconnect and reconnect the connector, listening for a 'click' to confirm it locks.
  • 3
    Harness continuity test: Disconnect the battery and the airbag connector. Replace the airbag unit with a dedicated airbag simulator (substitute resistor). Measure harness continuity from the seat connector to the SRS ECU plug. Inspect the harness sleeve at the seat slide rail mounting point for internal breaks.
  • 4
    Airbag unit resistance measurement: Power off the system. Use a dedicated airbag tester (or a 2Ω protection resistor in series) to measure the resistance across the airbag inflator terminals. Normal resistance is 2.0-3.0Ω. If the resistance is infinite or 0Ω, replace the left front seat side airbag assembly.
  • 5
    Connector terminal inspection and cleaning: Check connector terminals for oxidation, backed-out pins, or corrosion. Clean with electrical contact cleaner. Replace connector terminals or wiring harness assembly if necessary. Verify terminal contact pressure meets the standard (usually 0.5-1.5N).
  • 6
    System reset and verification: Reconnect all components, connect the battery, and use the diagnostic tool to clear the fault code. Perform an SRS system self-check (usually 6-8 seconds after turning the ignition switch ON). Confirm the fault code does not reappear. Perform a full-range seat adjustment test (slide forward and backward 10 times). Confirm again that the system generates no fault codes.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin EV450: Seat adjustment caused connector to come loose

✓
A 2018 Qin EV450 with 32,000 km: owner reports intermittent SRS warning light. Scan tool retrieved active DTC B1620-00. Inspection found the yellow connector beneath the driver's seat not fully latched, with only partial engagement of the locking tabs. Service history showed the seat was removed for carpet cleaning one week prior. Disconnected and reconnected the connector, ensuring both locking tabs fully engaged. Performed 20 full slide cycles of the seat; fault did not reappear. Root cause: connector misalignment during seat installation, causing intermittent connection despite appearing properly seated.
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD E1 seat runner chafed wiring harness

✓
2019 BYD e1, 58,000 km, SRS warning light stays on, DTC B1620-00. Initial inspection found connectors normal. Measured airbag resistance: 2.4 Ω (normal). Disconnected the ECU-end connector and checked the seat-to-ECU harness, finding an intermittent open circuit in one signal wire. Removed the seat and found the harness at the seat rail mounting bracket had three copper cores fractured internally from long-term chafing, though the outer insulation showed only minor wear marks. Replaced the seat harness assembly (P/N: C11-582XXX) and fitted anti-chafe sleeves at the harness mounting points, which resolved the fault.
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin 100 side airbag internal open circuit

✓
2017 BYD Qin 100. The SRS warning light came on after accident repairs, with DTC B1620-00. The vehicle had a previous side collision that damaged the left front door, but the airbags did not deploy. Inspection found the under-seat connector and harness continuity were normal. Using an airbag simulator in place of the seat airbag allowed the DTC to clear and the system to pass self-diagnosis, confirming the airbag itself had failed. Disassembling the seat airbag revealed the internal igniter bridge wire had cracked from the impact, causing poor contact. Replaced the left front seat side airbag assembly (including airbag cushion and inflator, part number SCE-582XXXX), then recoded and matched the module, resolving the fault.
BYD DTC AI AnalysisFrom Chinese market (translated)

E1 Connector corrosion on flood-damaged vehicle

✓
2020 BYD E1. SRS warning activated after driving through water. Retrieved DTCs B1620-00 and B1621-00 (both side seat airbags). Inspection found water had reached seat base level, with visible verdigris and oxidation inside the under-seat airbag connectors. Contact resistance measured 15Ω (normal: <1Ω). Cleaned the connectors using precision electronic cleaner and adjusted terminal clamping force with a specialised terminal repair tool, restoring normal resistance. Recommended the customer replace all water-affected harness connectors to prevent recurrence; customer opted for the cleaning repair. At three-month follow-up, no further faults had occurred.
BYD DTC AI AnalysisFrom Chinese market (translated)

Seat modification caused a fault in BYD Qin EV300

✓
2017 BYD Qin EV300. One week after the owner fitted racing seats, a B1620-00 fault appeared. Inspection revealed the modification shop had failed to properly connect the original seat side airbag harness to the new seats (which lack airbag functionality), causing the SRS ECU to detect an open circuit. Solution: Since the modified seats cannot be fitted with the factory airbags, we fabricated a simulator from a dedicated airbag resistor (2.2Ω, 1/4W) and connected it to the original harness connector. We performed the 'seat airbag deactivation' coding using the diagnostic tool (requires manufacturer authorisation), and advised the owner to sign a safety notice explicitly stating the loss of side airbag function after the modification.
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]