AU
B1798

This fault code indicates the SRS (Supplemental Restraint System) ECU detected the front passenger side second stage seat belt pretensioner circuit resistance fell below the system-calibrated threshold (typically below 1 — Seal U

Safety System

This fault code indicates the SRS (Supplemental Restraint System) ECU detected the front passenger side second stage seat belt pretensioner circuit resistance fell below the system-calibrated threshold (typically below 1.0Ω; normal range is 1.5–3.0Ω).

During a severe collision (high-speed or high-intensity impact), the second stage pretensioner provides a stronger restraining force than the first stage.

It typically integrates into the seat belt retractor or seat anchorage point.

Low resistance indicates a circuit short.

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

This fault triggers the SRS airbag warning lamp and may disable the front passenger side airbag and pretensioner functions.

This severely degrades collision safety performance and requires immediate repair.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Seat belt pretensioner internal squib short circuit: Damaged internal coil insulation causes an inter-turn short circuit, resulting in an abnormal drop in resistance.
  • 2Wiring harness under the seat or inside the B-pillar chafed or stripped: Frequent adjustment of the front passenger seat causes the wiring harness to rub against the seat frame or vehicle body metal edges, damaging the insulation and causing a short to ground.
  • 3Connector water ingress or corrosion: Vehicle wading, improper interior cleaning, or poor sealing causes oxidation or short circuits at the pretensioner connector pins (usually located under the seat or inside the B-pillar trim).
  • 4Wiring harness crushed or deformed: The seat slide rail adjustment mechanism pinches the wiring harness, or seat modifications or carpet installation crushes the harness, causing internal wires to contact and short circuit.
  • 5SRS ECU internal detection circuit fault: Abnormal control unit sampling resistor or A/D conversion circuit causes a false low resistance reading (less common; confirm only after ruling out wiring harness and components).
What To Do
  • 1
    Safety preparation: Disconnect the battery negative terminal and wait at least 90 seconds to fully discharge the SRS capacitor and prevent accidental airbag deployment during repair.
  • 2
    Fault Confirmation: Use VDS2000 or a dedicated BYD diagnostic tool to read the fault code. Confirm B1798 is a current fault (Active) rather than a history fault (History). Record the freeze frame data.
  • 3
    Visual inspection: Check the pretensioner connectors under the front passenger seat and at the B-pillar seat belt anchor for looseness, water ingress, corrosion, or bent pins. Check the wiring harness for obvious damage.
  • 4
    Resistance measurement: Disconnect the SRS ECU connector. Use a multimeter to measure the resistance of the front passenger second-stage pretensioner circuit pins (refer to the workshop manual for corresponding pin definitions). Simultaneously measure the resistance to ground to check for a short circuit.
  • 5
    Harness isolation test: Disconnect the pretensioner connector. Measure resistance on the harness side and the component side separately to determine if the short circuit is inside the pretensioner or in the harness.
  • 6
    Targeted repair: If the wiring harness has a short circuit, repair the damaged wire and rewrap the insulation, focusing on the moving section near the seat slide rail. If the pretensioner is faulty, replace the front passenger seat belt assembly (the pretensioner is usually not supplied separately).
  • 7
    System reset: Connect the battery, clear the fault code using the diagnostic tool, and perform the SRS system self-check (Sensor Check). Confirm B1798 does not return and the airbag warning light turns off.
Real-World Cases
BYD DTC AI Analysis

Yuan EV: Frequent seat adjustment chafed wiring harness, causing short circuit

✓
A 2019 BYD Yuan EV with 32,000 km on the clock displayed an airbag warning light on the instrument cluster. Scanning revealed DTC B1798 (front passenger second-stage pretensioner resistance low). Inspection found the wiring harness beneath the front passenger seat had chafed through at the slide rail, with damaged insulation causing a short to ground. On this model, frequent seat adjustment and retaining clips that work loose allow prolonged friction, leading to this failure. Repaired the damaged wiring, rerouted the harness, added corrugated tubing for protection, and repositioned the retaining clips. Cleared the fault.
BYD DTC AI Analysis

Tang DM pretensioner connector corroded after water fording

✓
2021 BYD Tang DM. Owner reported the airbag warning light stayed on after driving through water. Scan tool showed DTC B1798. Inspection found water and green corrosion inside the pretensioner connector beneath the front passenger seat, with abnormally low resistance between pins due to electrolyte conduction. Water entered through the sill trim while fording; the connector seal had aged and failed. Repair: Thoroughly cleaned the connector pins with electronic cleaner to remove oxidation, blown dry, applied conductive protective agent, and replaced the waterproof seal. Fault resolved.
BYD DTC AI Analysis

Song MAX seat modification caused wiring harness pinch

✓
2019 BYD Song MAX. The owner modified the front passenger seat to power adjustment, and the airbag warning light illuminated. DTC B1798 stored. Inspection revealed the seat frame pinched the pretensioner harness during the modification, causing internal conductors to short. The Song MAX has a short pretensioner harness; the installer did not leave sufficient slack. Repair: Repaired the damaged harness section, replaced the damaged pretensioner (the short may have caused internal damage), and rerouted the wiring to prevent strain throughout the seat's full range of movement.
BYD DTC AI Analysis

Qin Pro pretensioner internal squib fault

✓
2020 BYD Qin petrol, no accident history. SRS warning light suddenly came on. DTC B1798 intermittent; symptoms changed when wiggling the wiring harness. Initially suspected poor contact, but harness and connectors checked normal. Measured pretensioner resistance: intermittent 0.8Ω (low). Root cause: partial short in the pretensioner's internal igniter coil (component defect). Replaced front passenger seatbelt assembly (includes second-stage pretensioner). Fault eliminated.
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.