AU
B164A1B

This fault code indicates the Supplemental Restraint System (SRS) detected an open circuit or high resistance condition in the front passenger seat belt pretensioner circuit (resistance value outside the calibrated range of 2-5Ω) — Atto 8

Safety System

This fault code indicates the Supplemental Restraint System (SRS) detected an open circuit or high resistance condition in the front passenger seat belt pretensioner circuit (resistance value outside the calibrated range of 2-5Ω).

As a key actuator in the passive safety system, the seat belt pretensioner contains a pyrotechnic device.

During a frontal collision, electronic detonation generates a reaction force to rapidly tighten the seat belt and remove slack between the occupant and the belt.

Fault code B164A1B indicates the SRS control unit (ACU) cannot establish an effective electrical connection with the front passenger pretensioner.

This condition prevents the pretensioner from deploying during a collision, continuously illuminates the instrument cluster airbag warning light, and forces the system into a degraded protection mode.

Some models may also limit power output or disable other associated safety functions.

3
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Loose or disconnected pretensioner connector: Frequent seat adjustment or foreign object impact loosens the yellow dedicated connector (usually equipped with a shorting bar) under the front passenger seat. This is the most common cause on E2, E3, and Qin EV models.
  • 2Wiring harness mechanical damage: Long-term repeated movement of the seat slide rail wears through the harness insulation, or sharp edges on the seat frame cut the pretensioner circuit wiring, causing an intermittent open circuit.
  • 3Internal open circuit in the pretensioner assembly: Moisture, static electricity, or manufacturing defects cause an open circuit in the pretensioner igniter or bridge wire, resulting in infinite resistance.
  • 4Clock spring (slip ring) fault: If the pretensioner circuit routes through a rotary connector in the B-pillar or seat, a broken internal ribbon cable causes an intermittent open circuit.
  • 5SRS control unit connector corrosion: Oxidation, backed-out pins, or water ingress at the ACU module connector pins causes abnormal signal acquisition in the front passenger pretensioner circuit.
What To Do
  • 1
    Safety preparation: Disconnect the 12V battery negative terminal and wait at least 3 minutes to fully discharge the SRS system and prevent accidental airbag deployment during repair.
  • 2
    Fault Code Confirmation: Use a VDS2000 or Launch X431 diagnostic tool to access the SRS system. Read current fault code B164A1B and freeze frame data. Record the vehicle speed, timestamp, and ambient temperature at the time of occurrence. Confirm if this is a current hard fault.
  • 3
    Visual inspection: Remove the front passenger seat (keep the wiring harness connected). Check if the yellow pretensioner connector under the seat (usually marked 'PRETENSIONER' or 'SRS') is fully locked. Inspect the shorting bar inside the connector for deformation. Verify there are no signs of water or corrosion.
  • 4
    Resistance measurement: Disconnect the battery negative terminal and wait, then unplug the pretensioner connector. Use a multimeter to measure the resistance between the pins on the pretensioner-side connector. The normal value is 2.0-5.0 Ω (including harness resistance). If the multimeter reads OL (open circuit) or >10 Ω, the pretensioner body or wiring harness has an open circuit.
  • 5
    Wiring harness continuity test: If pretensioner resistance is normal, measure wiring harness continuity from the pretensioner connector to the SRS control unit (usually located under the center armrest or front bulkhead). Inspect the wiring harness sleeve near the seat slide rail for breaks, and check pin-to-pin and pin-to-ground insulation.
  • 6
    Component replacement and verification: If the wiring harness has an open circuit, repair and waterproof it. If the pretensioner has an open circuit, replace the seat belt retractor assembly (including the pretensioner). Reconnect all connectors and the 12V battery. Use the diagnostic tool to clear the fault code and perform the 'SRS system self-check' or 'configuration and calibration' procedure. Verify the fault code does not return and the warning light turns off.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Worn seat rails on the BYD E2 caused an open circuit in the pretensioner wiring.

✓
A 2019 BYD E2 with 32,000 km had an intermittent airbag warning light on the dashboard. VDS pulled DTC B164A1B (current). Inspection found the seat track pinched the harness under the front passenger seat when moved fully forward, wearing through the insulation and breaking the copper wires. Removing the seat showed the pretensioner connector was intact, but the harness had completely severed at the frame bend. Repair: cut out the damaged section, resoldered the wires, sealed the splices with waterproof heat-shrink tubing, added a thicker protective sleeve, and rerouted the harness to clear the seat track travel. Fault cleared; no recurrence at 3-month follow-up.
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin EV: Loose pretensioner connector causes intermittent open circuit

✓
A 2020 Qin EV owner reported the seatbelt warning light flashing intermittently. DTC B164A1B was stored as a history code. Inspection found the yellow double-lock 2-pin connector beneath the front passenger seat not fully seated — a gentle pull could detach it. A detailing shop probably disturbed it while cleaning the interior. The shorting bar inside the connector showed arc erosion from prolonged poor contact. Fix: Cleaned the connector terminals with electronic contact cleaner, applied conductive grease, re-locked the connector, and secured it with a cable tie. Performed an SRS system self-test, cleared the fault code, and the resistance reading stabilised at 3.2 Ω.
BYD DTC AI AnalysisFrom Chinese market (translated)

E3 front passenger pretensioner internal open circuit replacement case

✓
A 2020 BYD E3 had the airbag warning light on after accident repairs (front bumper replaced). Scan tool showed DTC B164A1B. Checked the pre-tensioner connector — no abnormality. Measured harness continuity from the connector to the SRS control unit — normal. Direct resistance measurement across the pre-tensioner body showed open circuit. Disassembled the front passenger seat belt retractor and found the pre-tensioner igniter had deployed during the impact but was not recorded (latent fault), or the replacement used part was already faulty. Resolution: Replaced with a new front passenger seat belt assembly (including pre-tensioner). Performed ‘Occupant Classification System (OCS) Calibration’ and ‘SRS Configuration Write’ under ‘Configuration and Calibration’ using VDS. Fault resolved.
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: 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]