AU
B1791

DTC B1791 indicates the SRS (Supplemental Restraint System) control module detects a circuit resistance of 0 Ω or near 0 Ω for the driver side seat belt second-stage pretensioner (Dual-stage Pretensioner) — Seal U

Safety System

DTC B1791 indicates the SRS (Supplemental Restraint System) control module detects a circuit resistance of 0 Ω or near 0 Ω for the driver side seat belt second-stage pretensioner (Dual-stage Pretensioner).

BYD SRS seat belt pretensioners typically feature a dual-stage ignition design.

During a severe collision, the second-stage pretensioner triggers sequentially or simultaneously with the first stage.

Normal pretensioner squib resistance measures 2.0-3.0 Ω.

A 0 Ω resistance indicates a short circuit (short to ground or internal squib short).

This causes the SRS to detect a pretensioner circuit fault, enter fail-safe mode, and continuously illuminate the instrument panel airbag warning light.

In extreme cases, the pretensioner fails to deploy or deploys unintentionally during a collision, severely compromising occupant restraint system effectiveness.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Driver seat belt pretensioner squib internal short circuit: Moisture or aging of the igniter charge or bridge wire inside the pretensioner causes an internal short to ground, resulting in zero resistance.
  • 2Under-seat wiring harness wear and short circuit: Frequent fore/aft adjustment of the driver seat chafes the pretensioner wiring harness insulation (usually routed along the seat slide rail). The exposed wire contacts body metal, causing a short to ground.
  • 3Water ingress or corrosion in the SRS ECU connector: Poor sealing of the SRS control module connector located under the centre console or floor allows water ingress after washing the vehicle or wading, causing a short circuit between terminals.
  • 4Pretensioner connector short circuit: Oxidation or foreign matter ingress in the pretensioner connector (located below the B-pillar or under the seat) causes a short circuit between the signal and ground terminals.
  • 5SRS control module internal fault: Damaged internal monitoring circuit falsely reports pretensioner resistance as 0. The actual pretensioner and wiring harness are normal.
What To Do
  • 1
    Safety preparation: Set the vehicle to OFF, disconnect the 12V battery negative terminal, and wait at least 90 seconds to fully discharge the SRS backup power supply and prevent accidental airbag deployment.
  • 2
    Fault Confirmation: Use a dedicated BYD diagnostic tool (VDS or ED400) to read the fault code. Confirm B1791 is a current fault (Active Code), not a history fault, and record the freeze frame data.
  • 3
    Visual inspection: Inspect the driver seat belt assembly (especially versions with the pretensioner body on the B-pillar or under the seat) for obvious external damage, burn marks, or water stains. Inspect the wiring harness under the seat for wear or crush marks.
  • 4
    Resistance measurement: Disconnect the connector between the SRS ECU and the pretensioner. Use a digital multimeter to measure the resistance between the two terminals on the pretensioner-side connector. The normal value is 2.0-3.0 Ω. Measure the resistance from each terminal to ground; it must be infinite. A reading of 0 Ω indicates an internal short circuit in the pretensioner. Replace the seat belt assembly.
  • 5
    Harness continuity check: Disconnect the pretensioner and SRS ECU connectors, measure continuity between the harness terminals, and check for a short to ground (damaged harness insulation shorting to ground) or a short to power (short to +B).
  • 6
    Connector inspection: Inspect the pretensioner connector terminals for push-out, deformation, or corrosion. Inspect the SRS ECU connector for signs of water ingress and check for short circuits caused by foreign matter between the terminals.
  • 7
    Replacement verification: If the wiring harness and connector are normal, perform a substitution test using a known-good pretensioner or jumper resistor (2.7Ω dedicated repair resistor). If the fault code clears, the original pretensioner is faulty.
  • 8
    Repair and verification: Replace the faulty component (seat belt assembly) or repair the wiring harness, reconnect all connectors, reconnect the battery, clear the fault code, perform an SRS system self-test, confirm B1791 does not return, and verify the airbag warning light turns off normally.
Real-World Cases
BYD DTC AI Analysis

Seat wiring harness chafing caused pretensioner short circuit in BYD Song Pro DM

✓
A 2020 Song Pro DM with 30,000 km displayed "Check SRS System" on the instrument panel. The diagnostic scan revealed DTC B1791 (driver second stage pretensioner resistance 0). Visual inspection of the B-pillar pretensioner showed no damage, and resistance measured 2.3Ω (within specification). Further inspection of the wiring harness beneath the seat revealed interference between the seat rail mounting bolt and the harness. The insulation had chafed through, causing the signal wire to short to ground. Repaired the wiring and rerouted the harness away from the bolt. Fault resolved.
BYD DTC AI Analysis

Tang EV SRS control module water ingress and corrosion after wading

✓
2021 Tang EV SRS warning light on after heavy rain flooding. DTC B1791. Inspection found vehicle submerged to seat height. SRS ECU beneath centre console; connector showed water ingress and terminal corrosion. Cleaned and dried terminals. Pretensioner circuit continuity normal. Cleared DTC, system normal. Check door seals and drain holes.
BYD DTC AI Analysis

Qin PLUS DM-i pretensioner connector terminal short circuit

✓
Customer reported the airbag warning light illuminated on their Qin Plus DM-i after replacing the driver's seat cover. Found DTC B1791. Inspection revealed the seat pretensioner connector at the seat base had been pulled during the cover replacement, displacing the internal metal terminals and shorting the signal terminal to ground. Repositioned the terminals and ensured the connector locked into place. Cleared the fault.
BYD DTC AI Analysis

Replaced Yuan EV pretensioner internal igniter due to short circuit

✓
2019 BYD Yuan EV developed DTC B1791 after accident repairs. Vehicle was in a front-end collision; first-stage pretensioner deployed but second-stage did not. Measured second-stage pretensioner resistance at 0.1Ω, indicating internal igniter short. Replaced driver-side seat belt assembly (including pretensioner), configured ECU and matched coding with diagnostic tool. Fault resolved; system self-test passed.
BYD DTC AI Analysis

Aftermarket seat heating modification caused SRS interference in Han EV

✓
A 2022 Han EV developed an intermittent B1791 fault after a third-party shop installed seat heaters. Inspection found the installer stripped the heating pad power wire and spliced it into the pretensioner harness, paralleling the heating element resistance (close to 0Ω) across the pretensioner circuit. The SRS ECU detected abnormally low resistance. Technicians removed the unauthorised wiring and restored the factory harness, clearing the fault permanently.
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.