AU
B1646

DTC B1646 indicates the SRS (Supplemental Restraint System) detects driver-side seat belt pretensioner circuit resistance below the calibrated threshold (typically <1 — Atto 8

Safety System

DTC B1646 indicates the SRS (Supplemental Restraint System) detects driver-side seat belt pretensioner circuit resistance below the calibrated threshold (typically <1.0Ω or near a short circuit).

The pretensioner contains an internal squib with a normal resistance between 1.5 and 3.0Ω (depending on specific vehicle calibration).

Low resistance indicates a short circuit risk.

Potential causes include an inter-turn short within the pretensioner igniter, a wiring harness short to ground, or abnormal connector continuity.

The SRS control unit classifies this fault as a high-risk condition, illuminates the airbag warning lamp, and may trigger the system self-protection mechanism (disabling the driver-side airbag and pretensioner).

This prevents the seat belt from retracting properly during a collision, severely compromising passive safety functions.

5
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Pretensioner internal igniter short circuit: Moisture, aging, or manufacturing defects damage the insulation layer of the igniter resistance wire inside the seat belt retractor, causing an internal short circuit.
  • 2Worn under-seat wiring harness: Frequent fore-and-aft adjustment of the driver's seat wears through the pretensioner wiring harness insulation (usually located under the seat or at the base of the B-pillar), causing contact with vehicle body metal and creating a short to ground.
  • 3Connector water ingress or corrosion: Vehicle wading, car wash fluid seepage, or electrolytic corrosion creates continuity between the internal terminals of the yellow SRS connector (with shorting bar), causing abnormally low resistance.
  • 4SRS control unit sampling circuit fault: Faulty internal ECU A/D conversion circuit or sampling resistor causes a false low resistance reading (actual circuit is normal).
  • 5Wiring harness damage from modifications: Improper routing when retrofitting seat heating, ventilation, or power adjustment functions causes the seat frame to crush and damage the harness, or mishandling pierces the SRS wiring harness.
What To Do
  • 1
    Safety preparation: Turn off the ignition switch, disconnect the negative battery terminal, and wait at least 90 seconds to fully discharge the SRS capacitor and prevent accidental airbag deployment.
  • 2
    Fault code analysis: Connect the BYD VDS or dedicated diagnostic tool. Read the B1646 freeze frame data and record the resistance value when the fault occurred (e.g., 0.3Ω, 0.8Ω) to distinguish between a hard fault and an intermittent fault.
  • 3
    Visual inspection: Check the yellow SRS wiring harness under the driver's seat and inside the B-pillar trim for damage, water stains, burn marks, or signs of third-party modification. Verify the connector locking tabs are intact.
  • 4
    Disconnect and isolate: Disconnect the pretensioner connector under the seat (Note: The connector has a built-in shorting bar; after disconnecting, measure the wiring harness side, not the pretensioner side). Measure the resistance between the pretensioner body terminals using a multimeter.
  • 5
    Component inspection: If the pretensioner unit resistance is <1.0Ω (standard: 1.5-3.0Ω), the pretensioner has an internal short circuit. Replace the driver seat belt assembly (including pretensioner). If the resistance is normal, the fault is in the wiring harness or connector.
  • 6
    Wiring harness continuity test: Measure the resistance to ground at both ends on the wiring harness side; it must be >1MΩ. Measure continuity between the wiring harness terminals and confirm no short to ground. Check the connector terminals for corrosion, deformation, or foreign matter.
  • 7
    Shorting bar check: Verify the connector shorting bar functions normally. When disconnected, it must automatically short the wiring harness side to prevent accidental ignition. A failed shorting bar may cause false fault reports.
  • 8
    Replacement verification: If the unit tests normal but the fault persists, install a known-good pretensioner to cross-verify and rule out a false ECU report.
  • 9
    Repair and Replacement: Repair the damaged wiring harness (solder and apply heat-shrink tubing; never use electrical tape) or replace the faulty pretensioner/seat belt assembly. Fully engage all connectors.
  • 10
    System reset and verification: Restore all connections, reinstall the battery, clear the fault codes, perform an ignition cycle test (ON-OFF-ON), confirm the SRS warning lamp turns off after the self-check, and perform a crash sensor simulation test (if supported) to verify normal system operation.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Internal short circuit in the pretensioner of a water-damaged vehicle

✓
A 2020 BYD Qin Pro's airbag warning light stayed on constantly after driving through water. Retrieved fault code B1646. Inspection found obvious water staining under the driver's seat. Removed and inspected the pretensioner connector; found mud and water deposits inside. Disconnected the pretensioner and measured its resistance at 0.2Ω (normal 2.0Ω). Water ingress caused a short circuit in the pretensioner's internal igniter. Replaced the driver seatbelt assembly. Resistance then measured 2.1Ω, back to normal. Cleared the fault code and the system returned to normal. Recommend checking the vehicle's wading depth and drain holes for blockages.
BYD DTC AI AnalysisFrom Chinese market (translated)

Seat modification crushed and damaged the wiring harness.

✓
A 2019 BYD Yuan EV developed a B1646 fault after the owner fitted an aftermarket seat ventilation system. Inspection revealed the modified wiring harness had been routed through the seat foam, interfering with the pretensioner harness. Seat movement compressed the harness, damaging the SRS harness insulation and causing a short to earth. Technicians repaired the harness by rerouting it away from moving parts and restored the original pretensioner connections. Resistance measurements returned to 1.8Ω and the fault cleared. When fitting aftermarket accessories, strictly avoid routing wires along SRS harness paths.
BYD DTC AI AnalysisFrom Chinese market (translated)

Electrolytic corrosion of the connector caused intermittent short circuits.

✓
A 2021 BYD Tang had an intermittent SRS warning light, DTC B1646 (intermittent). Inspection found green copper corrosion on the internal terminals of the yellow SRS connector under the driver's seat. Cleaning agent had seeped in during a previous interior cleaning, causing electrolytic corrosion. This corrosion created a micro-short between the terminals, with resistance fluctuating between 0.8 and 1.2 Ω. We cleaned the connector with electronic cleaner, applied conductive protectant, and replaced terminals as needed. The fault has not returned.
BYD DTC AI AnalysisFrom Chinese market (translated)

Internal short circuit in the pretensioner due to aging

✓
A 2018 BYD Song MAX developed a B1646 fault at 80,000 km, no water damage or modifications. Measured pretensioner body resistance at 0.5 Ω, diagnosed internal igniter short due to aging. Replaced the OEM driver's seat belt assembly (part number matched to VIN to ensure consistent resistance specifications), clearing the fault. Root cause: the pretensioner's internal resistance wire sustained long-term vibration stress, leading to insulation fatigue and an inter-turn short circuit.
BYD DTC AI AnalysisFrom Chinese market (translated)

SRS ECU sampling circuit false detection

✓
After battery replacement, a 2019 BYD Yuan set DTC B1646. Measured the seatbelt pretensioner resistance at 2.0Ω (normal) and found no short circuits in the wiring harness, but the DTC wouldn't clear. Suspected a calibration drift in the SRS ECU's internal sampling circuit after the power cycle. Disconnected the negative battery terminal for 30 minutes to reset the ECU, but the DTC remained. Replaced the SRS control unit (online matching and programming required), and the fault cleared. Such soft faults are rare; rule out all hardware issues before suspecting the ECU.
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]