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B1704

DTC B1704 indicates the Supplemental Restraint System (SRS) detects a communication circuit abnormality between the left curtain airbag (CAB) and the Airbag Control Unit (ACU) — Seal U

Safety System

DTC B1704 indicates the Supplemental Restraint System (SRS) detects a communication circuit abnormality between the left curtain airbag (CAB) and the Airbag Control Unit (ACU).

Specifically, the ACU detects the left curtain airbag squib circuit resistance falls outside the standard range (normally 2-3Ω), indicating an open circuit, short circuit, or poor connection.

As a safety-critical fault, this condition prevents the left curtain airbag from deploying during a side impact, severely compromising occupant head protection.

Potential causes include wiring issues, poor connector contact, or an internal squib failure within the curtain airbag assembly.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Left curtain airbag wiring harness connector loose, oxidized, or making poor contact: Commonly occurs at A-pillar and B-pillar connectors. Long-term vibration or damp environments cause terminal oxidation or loose retaining clips.
  • 2Hidden open circuit or wiring harness wear: Internal wire breakage in the A-pillar trim panel wiring harness due to repeated removal and installation or at bends, or roof wiring harness wear at the firewall pass-through.
  • 3Left curtain airbag unit fault: Open circuit or abnormal resistance in the internal igniter, typically caused by airbag aging or previous external impact.
  • 4Under-seat wiring harness crush damage: Sliding the front seat forward and backward crushes the airbag wiring harness underneath, causing partial wire breakage or insulation damage.
  • 5Airbag control module (SRS ECU) fault: ACU internal monitoring circuit fault or poor contact at the main connector causing a false curtain airbag connection fault.
What To Do
  • 1
    Safety preparation: Disconnect the negative battery cable and wait at least 3-5 minutes to ensure the SRS system capacitor discharges fully, preventing accidental airbag deployment.
  • 2
    Initial visual inspection: Check the curtain airbag wiring harness connectors on the left A-pillar, B-pillar, and roof for secure connection. Inspect the connectors for oxidation, corrosion, or signs of water ingress. Confirm full engagement of the locking tabs.
  • 3
    Circuit resistance measurement: Use a multimeter to measure the left curtain airbag circuit resistance. The standard value is 2-3 Ω. Infinite resistance indicates an open circuit; excessively low resistance indicates a short circuit.
  • 4
    Harness routing inspection: Remove the left A-pillar trim, headliner edge, and under-seat trim panel. Inspect along the harness routing for wear, crushing, or breaks. Focus on bends and mounting points.
  • 5
    Connector cleaning and treatment: If oxidation is present, clean the terminals with electrical contact cleaner and apply a dedicated conductive protectant. If the connector is damaged, replace the wiring harness connector assembly.
  • 6
    Component isolation test: If wiring measurements are normal, install a known-good left curtain airbag to determine if the curtain airbag itself is faulty.
  • 7
    System reset and calibration: Connect the battery, clear the fault code using the BYD dedicated diagnostic tool (VDS), and perform the SRS system configuration and calibration procedure.
  • 8
    Function check: Perform an SRS system self-check to confirm DTC B1704 does not return. Shake the wiring harness to verify there are no intermittent faults.
Real-World Cases
BYD DTC AI Analysis

Concealed break in BYD S6 internal A-pillar wiring harness causing B1704

✓
2011 BYD S6 Premium. Airbag warning light intermittently illuminated while driving. Scanner retrieved DTC B1704. The technician initially inspected the connectors and found them secure; however, the fault returned shortly after clearing the code. Removed the left A-pillar trim to investigate further. Found the curtain airbag wiring harness had suffered an internal conductor fracture at the bend inside the A-pillar due to long-term vibration, with no visible external damage. Resistance measurements intermittently showed normal values and an open circuit. Replaced the left curtain airbag wiring harness assembly, re-routed the harness inside the A-pillar, and fitted protective sleeving to prevent chafing against metal edges. The repair fully resolved the fault.
Original source ↗
BYD DTC AI Analysis

Disconnected connector after accident repair caused B1704 on BYD S6

✓
2013 BYD S6 airbag warning light stayed on constantly after front collision repair. Fault codes: B1704 (left curtain airbag disconnected) and B1705 (right curtain airbag disconnected). Inspection found the engine wiring harness had been replaced during the accident repair, but the SRS ECU connectors were not fully seated, and the left curtain airbag harness connector below the left B-pillar was not connected. Resolution: Reseated all SRS ECU connectors, ensuring the retaining clips fully engaged. Connected the left curtain airbag harness connector below the left B-pillar. Performed airbag system configuration and calibration with a diagnostic scanner. Cleared fault codes; fault resolved.
Original source ↗
BYD DTC AI Analysis

BYD S6 under-seat wiring harness crushed, triggering B1704

✓
The BYD S6's airbag warning light came on. DTC B1704 indicated the left curtain airbag was not connected, accompanied by faults for the driver and passenger seatbelt pretensioners not connected. The technician inspected the seatbelt pretensioners and curtain airbag connectors and found them visually normal, but measured fluctuating and unstable resistance in the left curtain airbag circuit. After removing the seat to inspect the airbag harness underneath, the technician found the harness had been crushed at the seat rail mounting point, with internal wires partially broken but not completely severed, causing intermittent poor contact. Resolution: Repaired the left curtain airbag wiring harness, replaced the damaged wire segments, re-secured the under-seat harness to prevent interference with the seat rails, cleaned all connectors, and applied conductive grease.
Original source ↗
BYD DTC AI Analysis

BYD S6 corroded connector causing intermittent B1704 fault

✓
2011 BYD S6 2.4L automatic, approximately 80,000 km travelled, with an intermittent airbag warning light. Diagnostic equipment retrieved DTC B1704, which cleared but returned within hours. Initial inspection found the left curtain airbag connector secure with no visible harness damage. Removed the left A-pillar trim for deeper inspection, and found slight oxidation on the connector pins and worn insulation on the wiring harness at the firewall penetration point. Solution: Cleaned the connector pins with electronic contact cleaner and applied dedicated conductive protectant, repaired the worn insulation at the firewall and added corrugated tubing protection, and replaced the left curtain airbag wiring harness connector assembly to prevent recurrence. Multiple post-repair test drives confirmed the fault did not return.
Original source ↗
BYD DTC AI Analysis

BYD S6 crash sensor connector not reconnected, causing B1704-related fault

✓
BYD S6 airbag warning light stays on. Retrieved DTC B1704 – Left curtain airbag disconnected. Owner reported a previous left door sheet metal and paint repair. Inspected left curtain airbag unit and connector – appearance normal. Used scan tool to read live data and found no signal from the left side impact sensor. Inspection found the side impact sensor connector inside the left B-pillar had been removed during the door repair and not fully reseated, causing a communication fault in the curtain airbag system. Solution: Reseated the left side impact sensor connector. Cleaned oxidized contacts on the B-pillar connector and tightened it. Performed airbag system self-test and configuration procedure using the scan tool. Cleared fault codes and system returned to normal.
Original source ↗
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.