AU
B164B

DTC B164B indicates the airbag control unit (SRS ECU) detects an abnormally low-resistance path (typically <1 — Seal U

Safety System

DTC B164B indicates the airbag control unit (SRS ECU) detects an abnormally low-resistance path (typically <1.5Ω) between the front passenger seat belt pretensioner ignition circuit and body ground (GND).

As a pyrotechnic safety device, the pretensioner igniter has a normal resistance of approximately 2.0-3.0Ω.

A short to ground causes the SRS system to detect a front passenger pretensioner circuit fault, illuminate the 'Airbag Fault Warning Light' continuously, and enter fail-safe mode.

During a collision, the system may fail to deploy the front passenger pretensioner and may subsequently disable the front passenger airbag.

This fault is a hard short to ground.

Repair immediately to ensure passive safety system integrity.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Damaged yellow SRS wiring harness sleeve under the front passenger seat; prolonged friction between the wire and the seat slide rail or body metal wears through the insulation, causing a short to ground.
  • 2Poor sealing of the pretensioner connector (usually located below the B-pillar or under the seat) allows water ingress after a car wash or wading, causing a short circuit between terminals or a short to ground.
  • 3Frequent forward and backward seat adjustment causes the seat rail to pinch the wiring harness, or a loose harness retaining clip allows the wire to contact a sharp edge on the vehicle body.
  • 4During accident repairs, the pretensioner wiring harness deviated from standard routing, directly contacting a metal point on the body.
  • 5Igniter coil insulation failure inside the pretensioner body (rare, usually accompanies previous deployment or aging).
What To Do
  • 1
    Safety preparation: Turn off the ignition switch, disconnect the 12V battery negative terminal, and wait at least 90 seconds to fully discharge the SRS capacitor and prevent accidental airbag deployment.
  • 2
    Fault Confirmation: Connect the BYD dedicated diagnostic tool (ED400 or VDS). Read the DTC B164B status to confirm it is an 'Active' fault rather than a history fault. Record the freeze frame data.
  • 3
    Visual inspection: Remove the front passenger seat (if necessary) and the lower B-pillar trim panel. Check the integrity of the yellow SRS wiring harness sleeve. Closely inspect the seat slide rail mounting points and the area where the harness passes through the floor for signs of abrasion, pinching, or water ingress.
  • 4
    Resistance measurement: Disconnect the pretensioner connector. Use a multimeter to measure the resistance between the harness-side (ECU-side) terminal and body ground. Normal value: >1 MΩ (infinite). A resistance of <1 Ω confirms a short to ground.
  • 5
    Section-by-section inspection: If the wiring harness shorts to ground, use the split-half method to progressively disconnect intermediate connectors and locate the short circuit (commonly found at harness bends under the seat or the B-pillar junction).
  • 6
    Pretensioner inspection: Measure the resistance across the two terminals of the pretensioner body. Standard value: 2.0-3.0 Ω (refer to the workshop manual). If the resistance is abnormal (<1 Ω or >5 Ω), replace the pretensioner assembly.
  • 7
    Repair and isolation: Repair the damaged wiring harness (use double-layer heat-shrink tubing for insulation). Re-secure the wiring harness to maintain >20mm clearance from the vehicle body metal. Install an anti-abrasion sleeve if necessary.
  • 8
    System reset: Reconnect all connectors, connect the battery, clear the fault code using the diagnostic tool, perform 'SRS system self-diagnosis', and confirm B164B does not return and the airbag warning light turns off.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Worn seat rails caused the wiring harness to short to ground.

✓
Vehicle: BYD Song MAX 2019. Symptoms: Dashboard airbag light stays on, DTC B164B set. Diagnosis: Removed the front passenger seat. The sharp edge of the seat rail had cut the yellow SRS harness sleeve underneath. The exposed wires contacted the metal rail directly, shorting to ground. Regular full loading and frequent passenger seat adjustments caused the harness to repeatedly bend and chafe at that spot. Solution: Repaired the damaged wire (replaced the entire harness section). Repositioned the harness mounting bracket. Ensured the harness clears all metal through the full seat travel. Fault cleared.
BYD DTC AI AnalysisFrom Chinese market (translated)

Connector shorted due to water ingress after wading

✓
Model: BYD Yuan EV 2019. Symptoms: Airbag warning light illuminated after driving in heavy rain; scan tool displayed DTC B164B. Diagnosis: Inspected the pretensioner connector beneath the passenger-side B-pillar and found the seal had deteriorated and was missing. Water stains and verdigris inside the connector caused the terminals to short to ground. Solution: Thoroughly cleaned the connector terminals, replaced the waterproof seal, used electronic cleaner to dry, applied dielectric grease, cleared the fault codes and restored normal operation.
BYD DTC AI AnalysisFrom Chinese market (translated)

Improper wiring harness routing after accident repairs

✓
Vehicle: BYD Tang 2021. Symptom: After front collision repairs, the instrument cluster displayed an airbag fault; DTC B164B was stored. Diagnosis: Inspection found that after replacing the front passenger seatbelt assembly, the technician had not secured the pretensioner harness with the factory clips. The harness was pinched between the seat bracket and floor pan, crushing the insulation and causing a short to ground. Solution: Rerouted the harness into the factory wiring channel, repaired the damaged insulation, added abrasion-resistant corrugated tubing, and ensured the harness had adequate free length to accommodate seat fore-aft adjustment.
BYD DTC AI AnalysisFrom Chinese market (translated)

Internal short circuit in pretensioner

✓
Vehicle: BYD Qin Petrol 2020. Symptoms: No collision history. Intermittent airbag warning light; DTC B164B intermittent. Diagnosis: Disconnected the pretensioner connector, measured the harness side – no short to ground. Measured resistance between the pretensioner terminals and housing, finding 0.8Ω low resistance (normal: open circuit). Determined the pretensioner internal igniter coil insulation had failed, causing a short to the housing. Solution: Replaced the front passenger seatbelt assembly (includes pretensioner). As the pretensioner is non-serviceable, replaced the entire assembly and performed recoding and matching.
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]