AU
B1799

DTC B1799 indicates the squib circuit resistance of the front passenger-side second-stage seat belt pretensioner (usually located under the front passenger seat or lower B-pillar) exceeds the SRS ECU threshold (standard value: 2 — Atto 8

Safety System

DTC B1799 indicates the squib circuit resistance of the front passenger-side second-stage seat belt pretensioner (usually located under the front passenger seat or lower B-pillar) exceeds the SRS ECU threshold (standard value: 2.0 Ω ± 0.4 Ω; typically triggers at >2.4 Ω).

This passive safety fault in the Supplemental Restraint System (SRS) means the ECU detects a high-resistance condition in the pretensioner circuit.

Poor contact, a partial open circuit in the wiring, or an aging internal pretensioner squib can cause this condition.

This fault may prevent the second-stage pretensioner from deploying correctly during a collision, reducing occupant restraint protection.

The SRS warning light remains illuminated to alert the driver.

5
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Loose pretensioner connector or oxidized terminal: The pretensioner plug under the front passenger seat or in the B-pillar area loosens due to vibration, or terminal surface oxidation or water ingress increases contact resistance (contact resistance exceeding 0.5 Ω causes total resistance to exceed the specification).
  • 2Partial open circuit or poor contact in the wiring harness: Prolonged bending or compression of the pretensioner wiring harness inside the B-pillar trim panel or near the seat slide rail partially breaks the internal copper strands, reducing the effective conductive cross-section and creating a high-resistance point.
  • 3Pretensioner internal squib aging: Age or environmental factors oxidize the internal resistance wire of the pretensioner squib, causing the resistance value to drift beyond the upper limit.
  • 4Poor ground circuit: The pretensioner circuit grounds to the vehicle body. Loose or corroded ground points, such as G301/G302, increase total circuit resistance.
  • 5Previous repair issue: Prior seat or B-pillar trim removal and installation left the pretensioner connector not fully locked or pinched the wiring harness in a body panel seam, causing hidden damage.
What To Do
  • 1
    Safety preparation: Turn off the ignition switch, disconnect the negative battery terminal, and wait at least 90 seconds to fully discharge the SRS capacitor and prevent accidental deployment.
  • 2
    Fault Confirmation: Connect the BYD VDS diagnostic tool, enter the SRS system to read fault codes, confirm B1799-00 and freeze frame data (record the specific resistance value), and check for associated fault codes.
  • 3
    Locate the component: Remove the front passenger seat or lower B-pillar trim panel, then locate the second-stage seat belt pretensioner (usually marked 'Pretensioner-2nd Stage').
  • 4
    Visual inspection: Check the pretensioner connector for looseness, the locking tab for breakage, the wiring harness for signs of crushing or abrasion, and the terminals for green oxidation or signs of water ingress.
  • 5
    Resistance measurement: Disconnect the pretensioner connector. Use a precision multimeter (accuracy 0.1 Ω) to measure the resistance between the two terminals on the pretensioner body. Standard value: 2.0 Ω ± 0.4 Ω (at 20°C). If >2.4 Ω, the pretensioner body is faulty. If normal, check the wiring.
  • 6
    Circuit test: Measure wiring harness continuity between the pretensioner connector and the SRS ECU. Check the harness resistance specifically at the B-pillar hinge (<0.5Ω). Inspect for high-resistance points or intermittent open circuits.
  • 7
    Ground check: Check pretensioner circuit ground points G301/G302. Clean the ground contact surfaces and tighten the bolts (standard torque typically 8-10 N·m). Ensure ground resistance is <0.1 Ω.
  • 8
    Repair/replace: If the connector is oxidized, clean with electrical contact cleaner and apply conductive grease; if the wiring harness is damaged, repair or replace the harness; if the pretensioner unit resistance exceeds the limit, replace with a genuine pretensioner assembly (Note: The pretensioner is an explosive component; handle the old part according to dangerous goods transport regulations).
  • 9
    Verification test: Reconnect all connectors and the 12V battery. Turn the ignition on. Use VDS to clear the fault code and perform the 'SRS system self-check'. Confirm B1799 does not reappear and the instrument cluster airbag warning lamp turns off. Conduct a road test to confirm no intermittent faults.
Real-World Cases
BYD DTC AI Analysis

BYD Qin Pro B1799 DTC – Oxidized Connector Causing High Resistance

✓
A 2019 BYD Qin Pro arrived with the airbag warning light constantly on. VDS retrieved DTC B1799-00. The technician removed and inspected the pretensioner connector beneath the front passenger seat and found visible green oxide inside—the vehicle had previous water exposure. The pretensioner measured 2.1 Ω (normal), but the harness side showed 1.2 Ω contact resistance. The technician cleaned the connector terminals with WD-40 electronic cleaner, applied conductive grease, and re-locked the connector. After clearing the DTC, the system self-test passed and the fault cleared.
Original source ↗
BYD DTC AI Analysis

BYD Song MAX B1799 Fault - B-Pillar Wiring Harness Crush Damage

✓
A 2020 Song MAX had an intermittently illuminated airbag warning light. The diagnostic scan showed code B1799-00 (historical). Inspection found signs the passenger side B-pillar lower trim had been removed (the owner had taken it off earlier for window tinting). Disassembling the B-pillar revealed the pre-tensioner harness trapped between the trim clip and sheet metal, damaging the insulation and partially fracturing the internal copper wires—about three strands remained connected. Resistance across this section measured 3.8Ω (normal is <0.5Ω). Repaired the wiring by cutting out the damaged section, soldering in an extension, and insulating with double-layer heat shrink tubing. Rerouted the harness to prevent pinching. Fault resolved.
Original source ↗
BYD DTC AI Analysis

BYD Tang DM B1799 – High Circuit Resistance from Poor Ground

✓
A 2021 Tang DM showed an airbag warning after accident repairs; VDS reported B1799-00. The vehicle had prior sheet metal work on the passenger-side A-pillar. The pretensioner connector checked normal and the unit measured 2.0Ω, but the negative circuit-to-body resistance was 2.5Ω (should be <0.1Ω). Traced to ground G302 under the passenger seat—the body shop never cleaned it during repairs, so paint residue caused poor contact. Fix: Sanded the ground point to bare metal and retorqued the bolt to 9N·m. Resistance dropped to 0.05Ω. Cleared the DTC; fault resolved.
Original source ↗
BYD DTC AI Analysis

BYD Yuan EV B1799 — Seatbelt pre-tensioner resistance drift due to aging

✓
A 2019 Yuan EV with 60,000 km had the airbag warning light on constantly, storing DTC B1799-00. Resistance at the pretensioner connector measured 3.2Ω (significantly high). Disconnecting the plug and measuring directly across the pretensioner body showed 3.1Ω (specification 2.0±0.4Ω), confirming internal igniter aging caused resistance drift. Wiring harness and connectors were normal. Replaced the front passenger second-stage seatbelt pretensioner assembly (Part No.: 5A-5826000-XX). The new part measured 1.9Ω. After installation, cleared the fault codes and the SRS self-test passed.
Original source ↗
BYD DTC AI Analysis

BYD Song Pro B1799 Intermittent Fault – Resistance Fluctuation Due to Loose Connection

✓
A 2020 Song Pro had an intermittent airbag warning light. VDS read historical fault code B1799-00 (current resistance 2.6Ω, borderline high). The pretensioner connector under the front passenger seat had a broken locking tab; the plug was not fully seated, causing an intermittent connection. Resistance fluctuated between 1.8Ω and 4.5Ω when moving the harness. Replaced the pretensioner connector (with wiring harness pigtail) and ensured full lock. Cleared the fault codes and road-tested continuously for 3 days; the fault did not recur.
Original source ↗
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.