AU
B177C

DTC B177C indicates a short to ground in the left middle-row seat belt pretensioner ignition circuit — Atto 3

Safety System

DTC B177C indicates a short to ground in the left middle-row seat belt pretensioner ignition circuit.

In the BYD SRS (Supplemental Restraint System) architecture, the pretensioner operates as a pyrotechnic device.

Its ignition circuit normally maintains a high-impedance state (normal resistance approx. 2-3 Ω).

The ECU determines a short to ground when it detects circuit voltage continuously below the threshold (<0.5 V) or an abnormal resistance drop (<1 Ω).

This fault causes: 1) left middle-row pretensioner failure, preventing normal deployment to tighten the seat belt during a collision; 2) the SRS to enter degraded mode, potentially affecting normal airbag deployment logic; 3) the ECU to cut circuit power to prevent accidental deployment from short-circuit currents.

This constitutes a severe active safety system fault.

Triggering this fault illuminates the airbag warning light and activates a continuous buzzer alarm.

The B177C fault code on the Atto 3 indicates a short to ground in the left middle-row seat belt pretensioner ignition circuit. Severity is high because the pretensioner may not deploy in a crash and the SRS may enter degraded mode, with the airbag warning light and buzzer active. Common causes include damaged under-seat wiring or water ingress and corrosion at the connector. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available.
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Cases Logged
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Causes
Likely Causes — Atto 3
  • 1Mechanical wear of the under-seat wiring harness: Frequent forward and backward sliding or folding of the left middle-row seat wears through the pretensioner harness insulation (usually located near the seat rail). This allows contact with the metal vehicle frame, creating a short to ground.
  • 2Connector water ingress and corrosion: During vehicle wading or interior cleaning, a poorly sealed pretensioner connector (located below the B-pillar or under the seat) allows moisture to enter, reducing insulation resistance between terminals or to ground.
  • 3Pretensioner internal short circuit: Manufacturing defects, aging, or external impact caused the internal squib coil to short to the housing.
  • 4Improper accident repair: Failing to correctly wrap the insulating tape when replacing the pretensioner or repairing the wiring harness after a vehicle collision causes the wire to contact the vehicle body ground.
  • 5Seat modification damage: Retaining screws pierce the pretensioner wiring harness during customer installation of seat covers, heating pads, or power adjustment mechanisms, causing a short to ground.
What To Do
  • 1
    Safety preparation: Disconnect the 12V battery negative terminal and wait at least 90 seconds (to fully discharge the SRS capacitor). Wear an anti-static wrist strap. Never use a multimeter resistance setting to measure the pretensioner connector directly (use a dedicated SRS resistance measuring tool or the diagnostic tool's active test function).
  • 2
    Visual inspection: Inspect the wiring harness sleeves in the left middle-row seat rail area, inside the lower B-pillar trim panel, and under the seat for damage, crushing, water marks, or burn marks. Focus on the wiring harness routing at the seat mounting bolts.
  • 3
    Disconnect the pretensioner: Unplug the left middle-row pretensioner connector (typically a yellow plug with a double-locking mechanism). Use the diagnostic tool to read the fault code status. If the code changes to an open-circuit fault (B177D), the short circuit is in the pretensioner unit. If it remains a short to ground, the short circuit is in the wiring harness.
  • 4
    Harness continuity test: Use a multimeter to measure continuity between both terminals of the pretensioner harness connector and vehicle body ground. The normal reading is infinity. If continuity exists, open the corrugated conduit section by section along the wiring to check for damage. Focus on the seat hinge movement area.
  • 5
    Pretensioner resistance measurement: Use the dedicated SRS test resistor (2Ω substitute resistor) or the diagnostic tool actuator test function to verify the pretensioner resistance is within the standard range (1.5-3.0Ω). If out of range, replace the pretensioner assembly.
  • 6
    Insulation repair: If the wiring harness is damaged, wrap the damaged area with double-layer heat-shrink tubing or high-temperature insulating tape. Reroute the harness to avoid areas of movement interference. Install a harness sleeve or cable ties if necessary.
  • 7
    Connector handling: If water enters the connector, clean the terminals with electrical contact cleaner, apply conductive grease, replace the sealing ring, and verify the locking tab is intact.
  • 8
    System reset: After repair, reconnect all connectors and restore the battery. Use VDS or Launch X431 to enter the SRS system and execute 'Configuration and Settings' → 'Seat Belt Pretensioner Check'. Clear the fault code and perform an ignition circuit self-check. Confirm the fault code changes to 'history fault' and the warning light turns off.
  • 9
    Function check: Perform a simulated crash test (use the diagnostic tool to execute the 'Crash Output Test' function; this only checks circuit continuity and does not actually deploy the pretensioner). Confirm the left second-row pretensioner circuit resistance and voltage parameters are within the normal range.
Real-World Cases
BYD DTC AI Analysis

Song MAX left second-row seat adjustment caused wiring harness wear and short circuit

✓
A 2019 BYD Song MAX with 32,000 km displayed "Check SRS System" on the dash. Scan revealed DTC B177C. Inspection found the wiring harness sheath torn behind the left middle-row seat rail, with two internal wires chafed through from frequent seat adjustment; one had shorted to the seat frame. Passengers regularly adjusted this seat through its full travel, causing excessive flexing at this point. Cut out the damaged section, soldered in extension wires, rerouted the harness to the outside of the seat rail clear of the movement path, and fitted a nylon protective sleeve. The fault has not recurred during three months of monitoring.
BYD DTC AI Analysis

Tang DM pretensioner connector corroded and shorted after driving through water

✓
2021 BYD Tang DM encountered heavy rain and drove through water deep enough to reach the door sills. The following day, the airbag warning light stayed on constantly. The scan tool displayed alternating codes B177C and B177B (short to power). Removed the left middle row seat and found water inside the pretensioner connector at the seat base crossmember; the terminals had oxidised and turned green. The seal ring had deteriorated, allowing water to enter under pressure during wading. Replaced the entire wiring harness plug including the pins, ultrasonic-cleaned the pretensioner socket with alcohol, sprayed WD-40 precision electrical contact cleaner, and applied waterproof silicone grease during reassembly. Also inspected the airbag connector on the same side with no faults found.
BYD DTC AI Analysis

Yuan EV pretensioner improperly installed after accident repair

✓
2019 Yuan EV. Left rear collision. Third-party shop replaced the left second-row pretensioner. One week post-repair, B177C set. Inspection showed the new pretensioner harness had pinched against a retaining clip where it passes through the B-pillar trim, damaging the insulation and shorting to earth. Also found: pretensioner mounting bolts torqued to only 15 N·m (spec: 35 N·m). Root cause: the unauthorised repairer skipped the workshop manual and omitted the dedicated harness clips. Fix: Replaced the damaged harness section (floor harness connector to pretensioner), retorqued the pretensioner to spec, and installed genuine clips to ensure harness routing complies with R&L (wiring diagram).
BYD DTC AI Analysis

Qin Pro heated seat modification inadvertently damaged pretensioner circuit

✓
2020 BYD Qin (petrol). Owner self-installed seat heating pads. While drilling into the left middle-row seat foam to secure the heating element, the installer penetrated the underside wiring harness with the drill bit, simultaneously damaging both pretensioner circuit wires and causing them to contact the seat metal frame. This triggered DTCs B177C and B177A (left middle-row pretensioner open circuit). Measured harness-to-ground resistance: 0.8Ω. Repair: Repaired the damaged harness (replaced the entire floor-side harness connector as the damage was too close to the connector), sealed splices with BYD-specific adhesive-lined heat-shrink tubing, and rerouted heating-pad wiring to the opposite side of the seat, away from the SRS harness. Reminded the owner to keep all modifications clear of airbag and pretensioner harness routing areas.
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.