AU
B164A-00

This DTC indicates the airbag control unit (SRS ECU) detects an open circuit or abnormal resistance in the front passenger seat belt pretensioner circuit — Seal U

Safety System

This DTC indicates the airbag control unit (SRS ECU) detects an open circuit or abnormal resistance in the front passenger seat belt pretensioner circuit.

Specifically, the ECU continuously monitors the pretensioner deployment circuit resistance via an internal bridge circuit (standard range: 1.6–3.0 Ω).

The ECU triggers DTC B164A-00 if it detects a resistance >6 Ω (open circuit) or <1 Ω (short circuit), or a communication interruption with the pretensioner.

As an active safety device, the pretensioner uses a pyrotechnic gas generator to generate tension within approximately 15 milliseconds of a collision, instantly retracting the seat belt webbing to eliminate slack against the occupant.

This fault indicates complete failure of this protective function.

Per the BYD SRS safety strategy, the system may enter a degraded mode (e.g., disabling side or curtain airbags), significantly increasing the risk of injury to the front passenger during a collision.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Loose or poor contact at the dedicated yellow SRS connector under the front passenger seat (most common; frequent forward and backward seat adjustment loosens or partially disconnects the plug).
  • 2Long-term fatigue of the pretensioner wiring harness at the seat slide rail bend breaks the internal copper strands (outer insulation intact but core wire broken, creating an intermittent open circuit).
  • 3Open circuit in the seat belt pretensioner internal resistance wire (aging, moisture ingress, or previous collision impact causing an internal open circuit in the pyrotechnic unit)
  • 4Liquid seeped under the seat during vehicle wading or interior cleaning, causing oxidation and corrosion of the SRS connector pins and increasing contact resistance.
  • 5Accidental disconnection of the SRS wiring during non-professional modification of the seat heating/ventilation system or seat cover replacement, causing terminal deformation, terminal back-out, or a crushed and damaged wiring harness.
What To Do
  • 1
    Safety preparation and initial diagnosis: Disconnect the negative battery terminal for 3 minutes to discharge residual system voltage. Use a dedicated BYD diagnostic tool (VDS or X-431) to read freeze frame data and record information such as vehicle speed and seat position when the fault occurred. Clear the fault code and road test the vehicle to confirm if the fault is intermittent.
  • 2
    Visually inspect the connector: Move the front passenger seat to the rearmost position. Inspect the yellow dedicated SRS connector under the seat (usually with a red locking tab). Verify the plug seats fully and the locking clip engages. Check inside the plug for water ingress, oxidation, or foreign matter.
  • 3
    Measure pretensioner resistance: Disconnect the connector. Use a multimeter to measure the resistance between the two terminals on the pretensioner side. The standard value is 1.6-3.0 Ω (at room temperature). If the reading shows infinity (OL) or >10 Ω, the pretensioner unit is faulty. Replace the front passenger seat belt assembly (the pretensioner usually integrates with the seat belt retractor; do not replace it separately).
  • 4
    Harness continuity test: Measure harness continuity between the connector under the seat and the SRS ECU (usually located beneath the centre tunnel). Focus on the harness at the seat slide rail bends. If an open circuit exists, repair or replace the harness. Also measure harness insulation resistance to ground and to the power supply to rule out a short circuit.
  • 5
    Check ground connection and ECU: Verify the SRS ECU ground point (usually located on the dashboard crossmember or under the center tunnel) is secure. Ground resistance must be <1Ω. If all above checks are normal, suspect an internal SRS ECU fault. Replace the control unit and perform coding and matching.
  • 6
    System reset and verification: Reconnect all components and turn on the power. Use the diagnostic tool to perform 'SRS system configuration' or 'seat position learning' (if applicable). Clear the fault codes. Perform static and dynamic tests. Confirm the airbag warning light turns off and the system has no fault codes.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Seat adjustment dislodged the connector on the Qin EV300.

✓
The airbag warning light was on when the vehicle arrived. The scan tool retrieved DTC B164A-00. Inspection found the front passenger seat adjusted fully forward to accommodate cargo; the yellow SRS connector locking tab under the seat was not fully engaged, leaving the connector partially seated. The connector had worked loose from frequent seat adjustments. Reseated the connector fully and locked the red retaining clip, then cleared the DTC to resolve the fault. Advised the customer not to place items under the seat that could interfere with the wiring harness.
BYD DTC AI AnalysisFrom Chinese market (translated)

Wiring harness fatigue fracture at Qin 80 seat rail

✓
The vehicle came in with the airbag warning light on. DTC B164A-00 appeared intermittently. Initial inspection showed normal connectors and the pretensioner resistance measured within specification. When repeatedly moving the seat to simulate operating conditions, the fault reappeared. Peeling back the corrugated tube on the outer side of the seat rail revealed two of the four internal wires had completely fractured due to long-term flexing. Replaced the front passenger seat wiring harness assembly (P/N: X60-5813300), rerouted the wiring and added a harness protective sleeve, completely resolving the fault.
BYD DTC AI AnalysisFrom Chinese market (translated)

Connector corrosion on Qin EV450 after water wading.

✓
Customer reported the airbag warning light illuminated after the vehicle drove through water. Inspection found the front passenger carpet damp, with obvious water staining and green copper corrosion inside the SRS connector beneath the seat. The pretensioner resistance measured normal, but the connector terminal contact resistance exceeded 5Ω. Cleaned the connector with electronic cleaner, blew it dry, and applied conductive grease. The fault temporarily cleared. However, it returned after one week. Eventually replaced the wiring harness connector and pretensioner (as moisture may have entered the pretensioner), and fixed the carpet waterproofing layer. This resolved the fault.
BYD DTC AI AnalysisFrom Chinese market (translated)

Aftermarket seat heater installation accidentally damaged SRS wiring

✓
Vehicle developed B164A-00 after installation of an aftermarket seat heating pad. Inspection revealed the installer pried open the SRS connector with a screwdriver during seat disassembly, bending the pins, and the heating pad crushed and damaged the wiring harness during seat installation. Straightened the pins and replaced the damaged harness section. Removed the unauthorized modification; it compromised SRS system safety. Restored original wiring layout. Cleared the fault code. The system returned to normal.
BYD DTC AI AnalysisFrom Chinese market (translated)

Internal resistance wire of pretensioner aged, causing an open circuit

✓
2017 BYD Qin 100, 80,000 km. Airbag warning light on. Inspected seat connectors and wiring harness – all normal. Direct measurement of pretensioner resistance showed infinite resistance. Disassembled seatbelt retractor, found pretensioner gas generator internal resistance wire fractured from long-term vibration fatigue. Replaced front passenger seatbelt assembly (including pretensioner), performed seatbelt pretensioner resistance configuration (some models require writing resistance value with diagnostic tool). Fault resolved.
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]