AU
B176C-00

This DTC indicates the airbag control unit (SRS ECU) detects the left rear seat belt pretensioner igniter resistance exceeds the calibrated threshold (normal range is typically 1 — Atto 3

Safety System

This DTC indicates the airbag control unit (SRS ECU) detects the left rear seat belt pretensioner igniter resistance exceeds the calibrated threshold (normal range is typically 1.6-2.4Ω; the DTC triggers when detected resistance is >3.0Ω or an open circuit occurs).

As a key actuator in the passive safety system, the pretensioner uses the igniter to detonate the gas generator during a collision, instantly tightening the seat belt to eliminate slack between the belt and the occupant.

Excessive resistance indicates high impedance or a potential open circuit in the igniter circuit.

This condition can prevent pretensioner deployment during a collision, severely compromising occupant restraint protection.

This is a hard DTC.

While the fault remains active, the system triggers a continuous warning and illuminates the airbag warning lamp.

Some models may also limit power output or disable specific assistance functions.

The B176C-00 fault code on the Atto 3 indicates high resistance or an open circuit in the left rear seat belt pretensioner igniter circuit. Severity is medium, as the pretensioner may not deploy correctly in a crash and the airbag warning light will stay on. Common causes include a loose or corroded pretensioner connector or damaged rear seat wiring. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
5
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Poor pretensioner connector contact: Vibration, oxidation, or a loose connection increases contact resistance at the left rear pretensioner plug (usually located inside the C-pillar trim or under the seat). This is the most common cause, especially after wading or exposure to damp environments.
  • 2Mechanical damage to under-seat wiring harness: For foldable rear seats, frequent folding can break internal copper strands or damage insulation at the bend of the pretensioner wiring harness (usually yellow), resulting in high resistance or an intermittent open circuit.
  • 3Pretensioner body aging: Oxidation of the internal igniter bridge wire or moisture ingress into the pyrotechnic charge causes resistance to drift outside the normal range over time. This typically occurs in vehicles over 5 years old or vehicles with prior exposure to high-temperature, high-humidity environments.
  • 4Open circuit in the SRS ECU to pretensioner wiring harness: Wear, rodent damage, or improper accident repair caused a partial break (not completely severed) in the body wiring harness within the B-pillar or floor cable channel.
  • 5Airbag control unit internal fault: A fault in the SRS ECU internal sampling circuit or analog-to-digital converter causes false high resistance readings. The probability is relatively low; rule out external wiring before confirming this fault.
What To Do
  • 1
    Safety preparation: Disconnect the 12V battery negative terminal and wait at least 90 seconds (or allow the capacitors to discharge completely as specified in the repair manual). Ensure the SRS system is completely powered down to prevent accidental pretensioner deployment during inspection.
  • 2
    Locate the pretensioner: Remove the left rear seat and C-pillar lower trim panel, find the left rear seat belt pretensioner assembly (usually integrated with the seat belt retractor), and identify the dedicated yellow wiring harness connector.
  • 3
    Visual inspection: Verify the pretensioner connector is fully seated and the locking tab is engaged. Inspect the connector pins for green oxidation, burning, or backed-out terminals. Check the wiring harness for signs of wear at the seat folding mechanism.
  • 4
    Resistance measurement: Use a dedicated SRS diagnostic ohmmeter (or a digital multimeter with current <1mA) to measure resistance across the pretensioner terminals. Normal resistance is 1.6-2.4Ω. If resistance is >3.0Ω, disconnect the connector and measure the pretensioner body and wiring harness side separately. If pretensioner body resistance is abnormal, replace the pretensioner. If wiring harness side resistance is infinite, check circuit continuity.
  • 5
    Harness continuity test: Measure harness continuity from the SRS ECU connector to the pretensioner connector. Standard resistance: <1Ω. Measure insulation resistance to ground. Standard resistance: >1MΩ. Carefully inspect the harness bend points under the seat and the C-pillar pass-through hole.
  • 6
    Substitution check: Connect a dedicated 3Ω simulation resistor (or a known-good pretensioner) to the circuit. Clear the fault code and observe if it returns to rule out ECU misreporting.
  • 7
    Repair/Replace: Based on measurement results, repair the wiring harness (soldering + heat-shrink tubing + corrugated conduit protection) or replace the pretensioner assembly. (Note: When replacing the pretensioner, record the old part number. After installing the new part, fully lock the connector and apply conductive grease to prevent oxidation.)
  • 8
    System reset: Reconnect all connectors and the battery. Use the diagnostic tool to clear the fault code. Perform the SRS system self-check. Confirm B176C-00 does not return. Verify the airbag warning light turns off. Finally, perform a road test to verify.
Real-World Cases
BYD DTC AI Analysis

Wear on the left rear seat folding mechanism caused the wiring harness to break internally.

✓
A 2018 Qin EV450 had the airbag warning light permanently on. DTC B176C-00. Found the left rear seat had been frequently folded for carrying cargo. Removing the seat revealed the pre-tensioner harness at the folding hinge had intact insulation but 90% of the internal copper strands were fractured, leaving only minimal contact and causing resistance to read 4.2Ω. Fix: Cut out the damaged harness section. Reconnected with 0.5mm² high-temperature wire using soldered joints. Protected the repair with double-layer heat shrink tubing and corrugated conduit. Rerouted the harness to avoid interference with the folding mechanism. Fault cleared.
BYD DTC AI Analysis

C-pillar connector oxidation causing increased contact resistance

✓
2017 BYD Qin 80. Owner reported intermittent SRS warning light, occasionally clearing after bumps. Scan tool showed stored DTC B176C-00 with resistance at 2.8Ω (threshold). Removed and inspected the left rear C-pillar pretensioner connector; found green oxidation on the terminals and a loose lock. Cause: Previous water wading allowed moisture into the C-pillar connector. Fix: Cleaned the terminals with electrical contact cleaner, applied copper-powder conductive grease, and replaced the connector lock (or the complete connector). Resistance measured 1.9Ω after tightening. Fault has not returned.
BYD DTC AI Analysis

Pretensioner unit resistance drift due to aging

✓
2017 BYD Qin EV300, 6 years old. Airbag warning light stayed on with DTC B176C-00. Infinite resistance at the pre-tensioner connector; 5.6 Ω measured directly across the pre-tensioner terminals—well above spec. Disassembled the pre-tensioner: internal igniter bridge wire oxidised and fractured. Natural ageing of the pre-tensioner, related to coastal high-humidity conditions. Replaced the left rear seatbelt pre-tensioner assembly (part number matched to VIN). Resistance normal after replacement. Coded and configured with the diagnostic tool (some models require writing the new pre-tensioner serial number). Fault resolved.
BYD DTC AI Analysis

Wiring harness connector not fully locked after accident repair

✓
A Qin 100 involved in a left-rear collision set DTC B176C-00 one month after 4S repairs. Diagnosis showed the pretensioner had fired in the crash (but went unrecorded), or the shop had only replaced the seatbelt assembly without fully locking the connector. In reality, the technician seated the plug during repair but missed the audible click; vibration worked it loose, causing poor contact of approximately 3.5Ω. Re-plugging the connector until the locking tab fully engaged cleared the fault immediately. After any SRS repair, run a 'pull test' to verify the connector locks.
BYD DTC AI Analysis

SRS ECU internal sampling circuit false positive

✓
2018 Qin EV450 repeatedly set DTC B176C-00. Measured pretensioner and wiring harness resistance: 1.8Ω (within spec), continuity good, no oxidation. Replaced the left rear seat pretensioner and wiring harness; fault persisted, appearing randomly at power-on. Found slight water ingress on SRS ECU connector pins from a windscreen leak dripping onto the ECU. Dried the connector, sealed the leak (lower windscreen trim panel), and cleared the codes. Monitored for one week with no recurrence. Confirmed intermittent ECU detection circuit fault. If the fault recurs, replace the airbag control unit.
Other Atto 3 Fault Codes
P0AA6Hybrid Battery Voltage System Isolation FaultB1108For 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.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.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.B173A1ADTC B173A1A indicates the right middle-row side airbag (typically located on the side of the right second-row seat or inside the C-pillar trim panel) igniter circuit resistance measures 0 Ω. In the Supplemental Restraint System (SRS), normal airbag igniter resistance ranges between 2.0 and 5.0 Ω. A resistance of 0 Ω indicates a short to ground or a short between circuit wires, preventing the Airbag Control Unit (ACU) from establishing a normal monitoring circuit. This safety-critical fault prevents the airbag from deploying during a collision, continuously illuminates the instrument panel SRS warning lamp, and may force the entire airbag system into fail-safe mode.
Common problems reported for Atto 3 →Atto 3 overview →
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.