AU
B1791-00

DTC B1791-00 indicates the driver-side Stage 2 Seatbelt Pretensioner resistance measures 0 ohms — Seal U

Safety System

DTC B1791-00 indicates the driver-side Stage 2 Seatbelt Pretensioner resistance measures 0 ohms.

Normal pretensioner igniter resistance is 2.0-4.0 ohms.

A 0-ohm reading indicates a circuit Short to Ground or an internal short in the pretensioner squib.

This fault prevents the SRS control unit from triggering the Stage 2 Seatbelt Pretensioner during a collision (typically used for secondary tightening in severe impacts), compromising occupant restraint system effectiveness.

Additionally, the SRS warning lamp remains illuminated, the system enters fail-safe mode, and the airbags may fail to deploy.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Pretensioner internal squib short circuit: Moisture, aging, or a manufacturing defect causes an internal short circuit in the pretensioner ignition device, dropping resistance to 0 ohms.
  • 2Wiring harness worn and shorted to ground: Long-term seat movement chafes and crushes the yellow SRS wiring harness under the driver's seat, shorting the wire to the vehicle body metal.
  • 3Connector water ingress and corrosion: Poor sealing of the seat pretensioner connector (usually located at the B-pillar or under the seat) allows water ingress after washing the vehicle or wading, causing a short circuit between pins.
  • 4ECU internal sampling circuit fault: A fault in the SRS control unit internal A/D converter or sampling resistor causes the system to incorrectly detect the pretensioner resistance as 0.
  • 5Improper repair procedure: failing to disconnect the battery during a previous seat belt assembly replacement caused a tool to short-circuit the pretensioner connector.
What To Do
  • 1
    Safety Preparation: Disconnect the 12V battery negative terminal and wait at least 90 seconds (to ensure the SRS capacitor discharges fully) to prevent accidental airbag deployment.
  • 2
    Fault confirmation: Use a dedicated diagnostic tool (such as BYD ED400 or Launch X431) to read the fault code and confirm B1791-00 is a current fault (Active), not a history fault.
  • 3
    Visual inspection: Check the yellow SRS wiring harness sleeve under the driver's seat for damage. Inspect the connector (usually marked D21 or D22) for looseness, water ingress, or green corrosion.
  • 4
    Resistance measurement: Disconnect the SRS ECU connector (located under the centre console or behind the centre armrest) and the pretensioner connector. Use a multimeter to measure the resistance on the pretensioner side. Normal resistance is 2-4Ω. A reading of 0Ω indicates an internal short circuit in the pretensioner.
  • 5
    Harness continuity check: Measure the resistance between the harness side of the pretensioner connector and body ground. The resistance must be infinite. A reading of 0 Ω indicates a short to ground in the harness. Strip the harness to locate the damaged point.
  • 6
    Insulation repair: If the wiring harness is damaged, wrap it with high-temperature insulating tape (≥125°C) or replace the SRS wiring harness assembly. If the connector has water ingress, clean it with electronic cleaner, blow dry, and apply conductive grease.
  • 7
    Component replacement: If the pretensioner resistance is 0 Ω, replace the driver seat belt assembly (including the pretensioner). Verify the resistance of the new part is within the standard range.
  • 8
    System verification: Reconnect all connectors and the battery. Use the diagnostic tool to clear the fault code. Perform an SRS system self-check (ignition switch ON; the indicator lamp should turn off). Perform a seat movement test to ensure the seat does not pinch the wiring harness.
Real-World Cases
BYD DTC AI Analysis

Qin EV450 seat movement caused wiring harness chafing and short circuit

✓
SRS warning light stayed on when the vehicle came in. Retrieved DTC B1791-00. Found the driver frequently adjusted the seat position, causing the seat rail edge to cut through the yellow SRS harness sleeve underneath. Exposed wires contacted the body and shorted to ground. Repair: Fixed the damaged harness (3 wires with damaged insulation, 1 completely severed), re-wrapped it with corrugated tubing, rerouted the harness, and added clips to prevent interference with the seat rails. Fault cleared after repair; no recurrence after 3 months of follow-up.
BYD DTC AI Analysis

Qin 80 pretensioner connector corroded after driving through water

✓
Customer reported the SRS warning light illuminated after heavy rain. DTC B1791-00 stored. Inspection confirmed water ingress; the carpet beneath the driver's B-pillar was damp. The pretensioner connector (D21) showed visible water stains and green oxidation. Resistance between pins measured 0.5Ω (short circuit). Diagnosis: Disassembled the connector and found the seal degraded. Cleaning the pins revealed electrolytic corrosion between pin 2 (ground) and pin 3 (signal) that created a micro-short. Repair: Replaced the connector assembly, treated the water-damaged harness section, applied waterproof silicone grease to the connector, and cleared the fault.
BYD DTC AI Analysis

Replaced rear seat belt pretensioner due to internal short circuit

✓
After accident repairs, the SRS warning light illuminated with DTC B1791-00. Service history showed the driver's seat belt assembly had just been replaced. The new pretensioner resistance measured 0.2Ω (abnormal); the old component measured 2.3Ω (normal) after disassembly. Root cause: The technician did not disconnect the battery during replacement. A wrench touched the pretensioner connector, causing momentary high current that burned out the pretensioner squib and created an internal short. Solution: Replaced the seat belt assembly under warranty and strictly followed SRS repair safety procedures (disconnect battery negative terminal first). Fault resolved.
BYD DTC AI Analysis

SRS ECU incorrectly detecting zero resistance

✓
Intermittent SRS warning light, DTC B1791-00. Static measurement showed pretensioner resistance of 2.8Ω (normal). Wiring harness insulation to ground tested good. Fault reoccurred on rough roads. Found loose connection at SRS ECU connector (inside dashboard) pin D21 (pretensioner signal). Oxidation caused unstable contact resistance; vibration triggered abnormal voltage sampling and the ECU misread it as 0Ω. Fix: Cleaned ECU connector pins, adjusted pin tension, applied conductive grease. No recurrence.
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.