AU
B1651

DTC B1651 indicates the Airbag Control Unit (ACU) detects the front passenger-side seat belt pretensioner circuit resistance exceeds the calibrated threshold (typically above 4 — Atto 8

Safety System

DTC B1651 indicates the Airbag Control Unit (ACU) detects the front passenger-side seat belt pretensioner circuit resistance exceeds the calibrated threshold (typically above 4.8Ω or open circuit).

The pretensioner utilizes a squib structure with a normal resistance of 2.0±0.3Ω.

Excessive resistance indicates a high-resistance or open-circuit risk in the firing circuit.

During a collision, this prevents the ACU from reliably triggering the pretensioner, leaving the seat belt unable to tighten promptly and severely compromising occupant protection.

This constitutes a Level 2 SRS fault.

The system illuminates the airbag warning lamp and may force the front passenger airbag into a degraded protection mode.

4
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Loose or oxidized wiring harness connector under the seat: Frequent forward and backward movement of the front passenger seat loosens the pretensioner wiring harness connector (usually located beside the seat rail or below the B-pillar), oxidizes the pins, or causes pin back-out, increasing contact resistance.
  • 2Pretensioner internal squib open circuit: The igniter cartridge bridge wire inside the seat belt pretensioner assembly breaks due to long-term vibration, moisture ingress, or manufacturing defects, resulting in high resistance or an open circuit.
  • 3Seat wiring harness broken: Repeated bending breaks the copper core of the pretensioner wiring harness in the transition area between the seat and the body (inside the wiring grommet), leaving only a few fine strands connected or severing the wire completely.
  • 4Airbag ECU internal sampling circuit fault: Damage to the sampling resistor, ADC conversion circuit, or driver chip inside the ACU for the front passenger pretensioner causes a false high resistance reading.
  • 5Third-party modifications: Installing seat heating pads, fitting non-genuine seat covers, or installing child seat anchorages crushed or damaged the pretensioner wiring harness and connector.
What To Do
  • 1
    Safety preparation: Disconnect the battery negative terminal and wait at least 90 seconds to fully discharge the airbag capacitor and prevent accidental pretensioner deployment.
  • 2
    Fault confirmation: Use the BYD VDS diagnostic tool to read the fault code. Confirm B1651 is a current fault (Active) rather than a history fault, and record the freeze frame data (ambient temperature, voltage, etc.).
  • 3
    Visual inspection: Check the pretensioner connectors (usually yellow plugs) under the front passenger seat and inside the lower B-pillar trim panel for looseness, water ingress, oxidized pins, or wiring harness damage. Carefully inspect the wiring protective rubber grommet near the seat rail.
  • 4
    Resistance measurement: Disconnect the pretensioner connector. Use a digital multimeter (low-current mode) to measure the resistance between the two wires on the pretensioner side. The normal value is 2.0 ± 0.3 Ω. If the resistance exceeds 4.8 Ω or is infinite, the pretensioner itself is faulty.
  • 5
    Harness continuity test: If pretensioner resistance is normal, measure harness continuity from the connector to the ACU. Inspect the harness within the seat movement range for breaks, high resistance (must be < 1Ω), or a short to ground.
  • 6
    Repair or replace: If the connector is faulty, clean the pins, apply conductive grease, and reconnect securely; if the wiring harness is broken, solder the break and apply waterproof insulation; if the pretensioner fails, replace with an OEM seat belt assembly (including the pretensioner). Never attempt to repair the pretensioner or measure the squib resistance.
  • 7
    System verification: Restore all connections. Turn on the power. Clear the fault code using VDS. Perform an SRS system self-check and confirm B1651 does not return. Perform a static test (simulate a crash signal) and a dynamic road test. Verify the airbag warning light remains off.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Frequent seat movement caused the connector to work loose

✓
A 2020 BYD Song MAX with 32,000 km had the airbag warning light on. Retrieved DTC B1651. Inspection found the front passenger seat was frequently moved back and forth (multiple family members used the vehicle). The pretensioner connector under the seat (yellow 2-pin plug) was not fully latched, leaving a gap of about 0.5 mm. The pin surfaces showed slight oxidation. Reseated the connector firmly, secured the wiring harness with a cable tie, and cleared the fault code. The issue was resolved. Advised the owner to avoid rough seat adjustment.
BYD DTC AI AnalysisFrom Chinese market (translated)

Seat modification crushed the wiring harness, causing internal wire breakage.

✓
A 2021 BYD Tang DM logged DTC B1651 one week after the owner installed an aftermarket passenger seat ventilation and heating pad. Removing the seat showed the heating pad mounting bracket had pinched the pretensioner wiring harness against the seat frame edge. The insulation was damaged, only 2-3 copper strands remained connected, and the resistance measured 12 Ω. The technician cut out the damaged section, soldered in replacement high-temperature wire of the same specification (0.5 mm², rated above 105 °C), sealed the joint with heat-shrink tubing for waterproofing, and cleared the fault. The technician advised the owner to avoid SRS wiring routes when modifying seats.
BYD DTC AI AnalysisFrom Chinese market (translated)

Seat belt pretensioner internal squib open circuit due to aging

✓
A 2019 BYD Yuan EV, four years old, logged DTC B1651. Checked the wiring harness connectors and found no faults. Measured the resistance at the pretensioner connector beneath the seat and found infinite resistance (open circuit). Removed the seatbelt assembly (located below the B-pillar) and found the internal pin of the pretensioner connector had detached from the igniter due to a failed solder joint caused by long-term vibration. Since the pretensioner is a single-use pyrotechnic device integrated with the seatbelt retractor, replaced the front passenger side seatbelt assembly (including pretensioner). Fault resolved.
BYD DTC AI AnalysisFrom Chinese market (translated)

Water ingress caused connector corrosion and high resistance

✓
My 2020 BYD Qin petrol logged a B1651 DTC after a blocked sunroof drain tube let water in below the passenger-side A-pillar. Checking under the seat, I found obvious water staining inside the pretensioner connector. The pins had verdigris and contact resistance measured 8.5 Ω. I cleaned the connector with electronic cleaner, sanded the pins with fine sandpaper, and applied dielectric grease to stop further oxidation. I also unblocked the sunroof drain tube, which fixed the fault. I recommend inspecting the sunroof drainage system regularly.
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]