AU
B173C

DTC B173C indicates the Supplemental Restraint System (SRS) detects the Right Second Row Side Airbag squib circuit resistance exceeds the calibrated threshold (typically >4 — Seal U

Safety System

DTC B173C indicates the Supplemental Restraint System (SRS) detects the Right Second Row Side Airbag squib circuit resistance exceeds the calibrated threshold (typically >4.5Ω).

This fault is an active diagnostic result of the SRS system, indicating a high-resistance condition in the circuit between the Airbag Control Unit (ACU) and the airbag module.

Causes for excessive resistance include increased contact resistance, a partially broken wiring harness, terminal oxidation, or an aging squib wire inside the airbag module.

This fault causes the system to mark the airbag circuit as 'unreliable', meaning the airbag may fail to deploy during a collision.

The fault typically illuminates the instrument cluster airbag warning light (SRS light) continuously, and some models also sound a warning buzzer.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Loose airbag module connector or oxidized terminals: Vehicle vibration, water ingress, or long-term oxidation increases contact resistance in the plug located under the seat or inside the C-pillar trim. This is the most common cause.
  • 2Wiring harness worn or partially broken: The right middle-row side airbag wiring harness routes through the seat slide rail or B/C-pillar area. Long-term bending and friction partially break the copper strands, reducing the effective conductive cross-section.
  • 3Internal airbag module fault: Igniter bridge wire aging, cold solder joints, or manufacturing defects causing internal resistance to drift outside the normal range of 2.0-3.0Ω.
  • 4Poor contact at the intermediate connector: Some models feature an intermediate connector between the body wiring harness and the seat wiring harness (usually located inside the sill trim panel). Terminal back-out or corrosion increases resistance.
  • 5SRS control unit sampling circuit fault: Internal ACU A/D converter or sampling resistor fault causes a false report of high resistance (confirm by substitution).
What To Do
  • 1
    Safe power-down: Turn off the ignition switch, disconnect the 12V battery negative terminal, and wait at least 3 minutes (or the time specified in the workshop manual) to fully discharge the SRS capacitor and prevent accidental deployment.
  • 2
    Locate the component: Confirm the physical location of the right middle-row side airbag (on MPV/SUV models such as the Song MAX and Tang, this typically mounts on the side of the right second-row seat backrest or inside the C-pillar trim panel). Remove the relevant trim panels or seat components to expose the yellow SRS connector.
  • 3
    Visual inspection: Check that the airbag module connector (usually marked 'SRS' or 'Airbag', yellow housing) is fully locked. Inspect the terminals for green oxidation, burn marks, or terminal back-out. Clean with electrical contact cleaner if necessary.
  • 4
    Resistance measurement: Measure the airbag circuit resistance using a dedicated airbag system tester (or a 2.2Ω current-limiting resistor in series). Disconnect the airbag module connector and measure the harness side: normal condition shows an open circuit or extremely high resistance (due to ACU internal resistance). Measure the module side: normal resistance is 2.0-3.0Ω. If the module side measures >4.5Ω, diagnose the module as faulty.
  • 5
    Segmented inspection: If the module side is normal, check wiring harness continuity. Use a multimeter to measure the wiring harness resistance between the ACU and the airbag connector; resistance must be <1Ω. Measure the insulation resistance to ground; resistance must be >1MΩ. Focus inspection on the wiring harness sleeve at moving parts under the seat for damage.
  • 6
    Repair verification: After repairing the wiring harness or replacing the airbag module, reconnect all connectors and the battery. Use the BYD ED400/ED600 diagnostic tool to clear the fault code, perform the 'SRS system self-check' procedure, and confirm DTC B173C does not return and the SRS warning lamp turns off.
  • 7
    Function check: During the final check, use the diagnostic tool to read the 'airbag resistance live data stream' and confirm the right middle-row side airbag resistance returns to the normal 2.0-3.0Ω range.
Real-World Cases
BYD DTC AI Analysis

2019 Song MAX: Right middle row seat airbag connector loose

✓
After driving through water, the SRS warning light stayed on constantly. Retrieved DTC B173C. Inspection found the airbag connector beneath the right middle-row seat (yellow 2-pin plug) was not fully locked, with visible water stains and oxidation inside. Fix: Cleaned the connector terminals with anhydrous alcohol, applied conductive grease, reinserted the plug firmly, and confirmed the locking tab was engaged. Cleared the DTC and resistance returned to 2.4Ω. Fault resolved.
BYD DTC AI Analysis

2021 Tang DM: C-pillar wiring harness wear caused high resistance

✓
Owner reported the SRS warning light occasionally illuminated after driving over bumps. Diagnostic scan revealed intermittent fault code B173C. Removed the right C-pillar trim panel and found the airbag wiring harness sheath damaged where it passes through the body sheet metal, with 3 of 5 internal copper strands fractured. This resulted in normal resistance when measured cold, but resistance fluctuated above 5 Ω during vehicle vibration. Repair: Cut out the damaged harness section and spliced in high-temperature wire of the same specification, sealed with heat shrink tubing, and secured the harness routing to prevent further chafing.
BYD DTC AI Analysis

2019 BYD Yuan EV airbag module internal aging

✓
Accident-repaired vehicle (right side scrape). Replaced the right second-row side airbag, and code B173C set. Measured resistance of the old module at 6.8Ω (damaged) and the new module at 2.2Ω (normal), but the fault persisted after installation. Further inspection showed the Airbag Control Unit (ACU) sustained impact damage in the accident, damaging the internal sampling circuit. Solution: Replaced the SRS control unit and recoded. Fault cleared.
BYD DTC AI Analysis

Qin Pro petrol: Door sill wiring harness connector corrosion

✓
Vehicle in a humid southern region. SRS light stays on with DTC B173C. Inspection found water ingress at the intermediate connector inside the right front sill trim panel (connecting body harness to seat harness) due to aged sealant, causing terminal verdigris. Replaced the entire connector assembly (including terminals and seals), re-crimped the wires, and applied waterproof sealing.
BYD DTC AI Analysis

2020 Song MAX seat modification stretched the wiring harness

✓
One week after fitting aviation seats, the SRS warning light illuminated. Inspection found the modification had excessively stretched the right middle-row side airbag harness, loosening the terminal-to-wire crimp inside the connector. Resistance fluctuated between 3–8 Ω when measured. Repair: Cut off the damaged connector and replaced it with a genuine airbag harness connector. Re-crimped and soldered the connections, ensuring the harness length allows full seat travel.
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.