AU
B16401B

DTC B16401B indicates the Airbag Control Unit (ACU) detects the driver-side seat belt pretensioner circuit resistance falls outside the normal range (typically 2 — Atto 8

Safety System

DTC B16401B indicates the Airbag Control Unit (ACU) detects the driver-side seat belt pretensioner circuit resistance falls outside the normal range (typically 2.0-3.0 Ω), indicating an open circuit or disconnected state.

This active SRS fault means the driver-side seat belt pretensioner may fail to ignite and retract during a collision, increasing seat belt slack and reducing occupant restraint protection.

The '1B' fault code suffix typically represents a specific subtype high-byte/low-byte coding, specifically denoting an open circuit or high-resistance fault.

This fault illuminates the airbag warning light continuously.

Some models may sound a warning buzzer and force the airbag system into a degraded protection mode, allowing only partial airbag deployment.

5
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Loose or poor contact at the under-seat pretensioner wiring harness connector: Frequent seat adjustments and vibration may loosen the yellow pretensioner connector near the seat rail (usually located on the inner side or underside of the seat), or pin oxidation may increase contact resistance.
  • 2Seat belt pretensioner internal open circuit: Moisture in the pretensioner gas generant, an aging and broken igniter tube, or an open internal bridge wire breaks the circuit.
  • 3Seat wiring harness worn and broken: As the seat slides forward and backward, the pretensioner wiring harness (especially sections routed through the seat rails or under the carpet) bends repeatedly over time, causing fatigue fractures in the internal copper wires while the outer insulation remains intact.
  • 4Airbag Control Unit (ACU) internal fault: A damaged sampling resistor or MOSFET switch in the ACU internal pretensioner drive circuit causes the ACU to misjudge the pretensioner status.
  • 5Vehicle wading or interior cleaning causes connector corrosion: water ingress under the driver's side carpet (e.g., blocked sunroof drain or A/C condensate leak) causes oxidation and green corrosion on the pretensioner connector pins under the seat, resulting in high resistance.
What To Do
  • 1
    Safety preparation: Switch the vehicle OFF, disconnect the low-voltage battery negative terminal, and wait at least 3 minutes to fully discharge the SRS capacitor and prevent accidental airbag deployment.
  • 2
    Visual inspection: Remove the driver's seat (or lift only the front of the seat) and verify the yellow pretensioner connector under the seat (usually marked 'SBT' or 'D-PRET') is fully connected. Inspect the connector latch for breakage and the pins for oxidation or backing out.
  • 3
    Wiring harness inspection: Inspect the pretensioner wiring harness sleeve along the seat rail for wear or pinch marks. Focus on the bending points during forward and backward seat movement. If necessary, open the corrugated conduit to inspect the internal wires.
  • 4
    Resistance measurement: Use a multimeter to measure the resistance between the pretensioner connector terminals. The normal value is 2.0-3.0 Ω (1.8-2.5 Ω on some models). Infinite resistance indicates an internal open circuit in the pretensioner; unstable resistance indicates poor connector contact.
  • 5
    Harness continuity test: Disconnect the ACU connector. Measure harness continuity between the pretensioner plug and the ACU. Resistance must be less than 1 Ω. Measure insulation resistance to ground; resistance must be greater than 10 MΩ (to rule out a short to ground).
  • 6
    Component replacement: If inspection confirms an internal open circuit in the pretensioner, replace the driver seat belt assembly (the pretensioner is usually integrated with the seat belt retractor). If the wiring harness is damaged, repair or replace the wiring harness. If the connector is oxidized, clean it and apply conductive grease. Replace the connector if necessary.
  • 7
    System reset: Reconnect all components and the battery. Use a diagnostic tool (such as BYD VDS or Launch X-431) to perform 'SRS System Configuration' and 'Seat Position Sensor Calibration' (if equipped). Clear the fault codes and perform a static self-test.
  • 8
    Dynamic verification: Perform three ignition cycles (OFF-ON-OFF), confirm the fault indicator turns off, and read the data stream to verify the pretensioner resistance value fluctuates within the normal range.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

E2 ride-hailing vehicle: loose seat connector causing intermittent warning

✓
A 2019 BYD E2 ride-hailing vehicle with 80,000 km had an intermittent airbag warning light on the instrument cluster, occurring more frequently on rough roads. VDS scanning returned DTC B16401B, with the fault history logged as intermittent. Inspection revealed the driver's seatbelt pretensioner connector (yellow two-pin plug) had a broken locking tab and was loose. Frequent seat adjustments for passengers entering and exiting the vehicle caused the connector to work loose over time. The repair involved replacing the pretensioner connector assembly (including wiring harness repair section) and securing the harness to the seat frame mounting points with cable ties to stop the seat movement from pulling on the harness. The fault did not recur during a one-month follow-up.
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin EV pretensioner not connected after accident repair

✓
2020 BYD Qin EV. After front-end collision repairs, the airbag warning light stays on. Stored DTC: B16401B (current fault). Inspection found the driver's pretensioner had deployed (needs replacement). The repair shop replaced the seatbelt assembly but failed to fully seat the pretensioner connector (audible click but pins not engaged). Reconnected the connector, heard the secondary lock engage, cleared the DTC – but it returned. Further measurement showed new pretensioner resistance 0.8 Ω, below spec. The aftermarket component was out of specification. Replaced with genuine seatbelt assembly; resistance measured 2.3 Ω. Fault fully resolved.
BYD DTC AI AnalysisFrom Chinese market (translated)

E3: High resistance in connector due to corrosion after wading

✓
2020 BYD E3. After driving through water about 30 cm deep in heavy rain, the airbag warning light came on. Scanned DTC B16401B. Live data showed pretensioner resistance at 5.6 ohms (out of spec). Removed the driver's seat and found the carpet damp. The pretensioner connector had green copper corrosion inside. Cleaned the terminals and applied electrical contact cleaner; resistance dropped to 2.1 ohms, but the fault returned the next day. Finally, replaced the connector, cleared the standing water from the carpet (unblocked the sunroof drain tube), and wrapped the connector with waterproof tape. Permanently resolved the fault. Advised the owner to avoid driving through water and to check drain holes regularly.
BYD DTC AI AnalysisFrom Chinese market (translated)

Wiring harness chafed through at seat rail, open circuit

✓
My 2019 Qin EV has 120,000 km on the clock. The airbag warning light stayed on constantly with DTC B16401B. I checked the connectors and pretensioner resistance—both normal at 2.2Ω—but wiggling the seat harness caused the resistance to intermittently go to infinity (open circuit). I stripped back the corrugated tubing near the seat rail and found the seat rail pinched the pretensioner wiring (yellow/black) when the seat was moved fully forward or backward. Long-term friction broke the internal copper strands, leaving only the insulation intact. I rerouted the wiring from the ACU, using dedicated wire loom protection to bypass the rail’s travel path and prevent mechanical interference. This is a common issue on the Qin EV and E2; the manufacturer should improve the harness routing.
BYD DTC AI AnalysisFrom Chinese market (translated)

ACU sampling circuit misreported fault

✓
A 2020 E3 had persistent DTC B16401B after multiple replacements of the driver's seatbelt and wiring harness. The pretensioner resistance and harness continuity were normal, but live data showed the ACU detected an open circuit. Inspection of the ACU connector revealed terminal expansion on Pin 23 (pretensioner high-side drive). Tightening the terminal with a harness repair tool did not resolve the fault. Technicians determined the fault to be an internal pretensioner drive circuit issue in the ACU (likely sampling resistor drift). Replacing the Airbag Control Unit (ACU) and recoding and matching it completely resolved the fault. This case reminds technicians that after ruling out external wiring and components, they should consider the control unit itself as the source of the failure.
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]