AU
B1794

DTC B1794 indicates the SRS (Supplemental Restraint System) ECU detects an open circuit (infinite resistance) in the driver-side seat belt pretensioner stage 2 firing circuit — Atto 8

Safety System

DTC B1794 indicates the SRS (Supplemental Restraint System) ECU detects an open circuit (infinite resistance) in the driver-side seat belt pretensioner stage 2 firing circuit. "Stage 2" means the pretensioner uses dual-stage firing technology, triggering in stages based on collision severity. "Disconnected" means the ECU cannot detect the firing circuit's normal resistance (standard value: 2.0–3.0 Ω).

During a moderate or severe collision, the pretensioner may fail to execute the stage 2 tensioning, reducing occupant restraint protection.

Typical causes for this critical passive safety system fault include an open wiring harness, a disconnected connector, a damaged clock spring, or an internal open circuit in the pretensioner.

3
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Internal open circuit or poor contact in the clock spring (spiral cable) under the steering wheel, interrupting the signal to the seat belt retractor pretensioner.
  • 2Loose connection, backed-out pins, or corrosion at the pretensioner connector under the driver's seat or at the B-pillar, causing an open circuit.
  • 3Open circuit in the internal igniter of the seat belt pretensioner assembly (possibly due to reaching the deployment threshold or a manufacturing defect)
  • 4Wiring harness between the seat slide rail and body fractured due to fatigue from long-term seat adjustment.
  • 5SRS ECU internal detection circuit fault incorrectly reporting pretensioner connection status
What To Do
  • 1
    Use the VDS2000/VDS3100 diagnostic tool to read all SRS fault codes, confirm whether B1794 is an Active or History fault, and record the freeze frame data.
  • 2
    Disconnect the battery negative terminal and wait 90 seconds to fully discharge the SRS capacitor. Inspect the pretensioner connectors under the driver's seat and inside the B-pillar trim panel for looseness, oxidation, or water ingress. Clean and retighten if necessary.
  • 3
    Check the continuity of the clock spring (spiral cable) under the steering wheel. Measure the continuity between the pretensioner-side wiring harness and the clock spring. If the fault code occurs intermittently when turning the steering wheel, replace the clock spring first.
  • 4
    Remove the driver-side seat belt retractor and measure the resistance between the two terminals of the pretensioner squib. Normal resistance is 2.0-3.0 Ω. If the reading shows OL (open circuit), replace the seat belt assembly (do not repair the pretensioner separately).
  • 5
    Check wiring harness continuity between the SRS ECU and the pretensioner. Inspect the harness sleeve near the seat slide rail for wear. After repairing the open circuit, perform a functional test using a 2Ω simulator resistor.
  • 6
    After completing the repair, reconnect the battery, clear the fault codes, and perform an SRS system self-check to confirm B1794 does not return. Finally, perform a simulated crash test (using the diagnostic tool component test) to verify normal pretensioner circuit continuity.
Real-World Cases
BYD DTC AI Analysis

Tang DM clock spring open circuit causing intermittent B1794 fault

✓
A 2021 Tang DM with 30,000 km displayed an intermittent "Check SRS System" warning on the instrument cluster. The scan tool retrieved DTC B1794 (current fault). The fault appeared when turning the steering wheel to specific angles. Removing the steering wheel revealed broken internal wiring in the clock spring, causing an open circuit in the driver's side second-stage pretensioner loop. After replacing the clock spring, pretensioner resistance measured 2.3Ω and the fault cleared. Prolonged full-lock steering caused excessive torsional fatigue in the clock spring.
BYD DTC AI Analysis

Loose connector under Yuan EV seat triggers safety system warning

✓
A 2019 BYD Yuan EV displayed DTC B1794 after routine maintenance. Service history showed the driver's seat was removed and refitted during the previous service. Inspection of the wiring harness connector beneath the seat found the pretensioner plug not fully seated, with the locking tab unengaged. After reconnecting and securing the harness, the fault code cleared and did not return. This type of fault commonly occurs after seat removal and refitting; make sure the yellow SRS plug fully engages with an audible click.
BYD DTC AI Analysis

Qin Pro Pretensioner Body Internal Open Circuit Replacement Case

✓
2020 BYD Qin petrol model, accident repair. After replacing the front bumper, DTC B1794 remained active and would not clear. Continuity from the SRS ECU to the seat connector checked normal, but the pretensioner itself showed infinite resistance. Disassembling the seatbelt retractor revealed the pretensioner had fired second stage during the accident with no external rupture signs (internal igniter blown). Replaced the driver-side seatbelt assembly (including pretensioner); fault cleared. Accident repairs must include measuring pretensioner resistance; never rely on visual inspection alone.
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.