AU
B177C

DTC B177C indicates a short to ground in the left middle-row seat belt pretensioner ignition circuit — Atto 8

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 8 indicates a short to ground in the left middle-row seat belt pretensioner ignition circuit. This is a high-severity SRS fault that can stop the pretensioner working and may affect airbag system logic. Common causes include damaged under-seat wiring from seat movement or moisture and corrosion in the connector. Visit an authorised dealer for diagnosis and repair. Typical repair cost data is available from recorded cases.
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Cases Logged
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Causes
Likely Causes — Atto 8
  • 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 wiring harness insulation (usually located near the seat rail), causing the harness to contact the metal frame and create 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 the terminals or to ground.
  • 3Internal short circuit in the pretensioner unit: Manufacturing defects, aging, or previous external impact cause the internal squib coil to short to the housing.
  • 4Improper accident repair: Failing to correctly wrap insulating tape when replacing the pretensioner or repairing the wiring harness after a collision causes the wire to contact body ground.
  • 5Seat modification damage: When the customer installs seat covers, heating pads, or electric adjustment mechanisms, retaining screws pierce the pretensioner wiring harness and cause a short to ground.
What To Do
  • 1
    Safety preparation: Disconnect the 12V battery negative terminal and wait at least 90 seconds (to allow the SRS capacitor to discharge fully). Wear an anti-static wrist strap. Never use a multimeter to measure resistance directly at the pretensioner connector (use a dedicated SRS resistance measuring tool or the diagnostic tool 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, pinching, water stains, 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 (usually a yellow plug with a double-locking mechanism). Read the fault code status using the diagnostic tool. If the status changes to an open-circuit fault (B177D), the short circuit is in the pretensioner assembly. If the status 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. Normal reading is infinity. If continuity exists, open the corrugated conduit section by section along the wiring to inspect 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 body resistance is within the standard range (1.5-3.0Ω). If out of range, replace the pretensioner assembly.
  • 6
    Insulation repair: After identifying harness damage, wrap the damaged area with double-layer heat-shrink tubing or high-temperature insulating tape. Reroute the harness to avoid interference with moving parts. Install a harness sleeve or securing cable ties if necessary.
  • 7
    Connector handling: If the connector has water ingress, 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 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 verification: Perform a simulated crash test (use the diagnostic tool to execute the 'Crash Output Test' function to check circuit continuity only, without actual deployment). Verify the left middle-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 8 Fault Codes
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.B110A02DTC B110A02 indicates the thermal management master control unit (HVAC ECU) or body control module (BCM) failed to receive valid CAN data frames from the PM2.5 air quality sensor over consecutive communication cycles (typically 500ms per cycle). The sensor typically mounts at the HVAC unit air inlet or behind the glove box and transmits real-time particulate concentration data (unit: μg/m³) to the HVAC controller via the CAN bus. The fault is essentially a communication link layer timeout. Potential causes include physical layer wiring faults, sensor power supply failures, sensor internal MCU lockup, or CAN network topology damage. Although this non-critical fault does not directly affect driving safety, it disables the intelligent air purification strategy and the automatic fresh air/recirculation switching function. In extreme cases, it triggers a thermal management system degraded mode, limiting compressor power or fixing the air flap position.B110B07This fault code indicates abnormal operation of the internal air pump (sampling pump) in the PM2.5 air quality detection module. The PM2.5 detector uses the laser scattering principle to measure particulate concentration and requires a built-in air pump to draw ambient air into the detection chamber at a constant flow rate. The ECU sets this code when it detects an open or short in the air pump drive circuit, abnormal current (stall or free-running), or abnormal speed feedback. This fault causes the automatic air conditioning system to lose air quality sensing capability, preventing automatic switching between fresh air and recirculation modes. The instrument cluster air quality display may show '--' or a fixed value, but the fault does not affect the vehicle powertrain or basic air conditioning cooling and heating functions.B173CDTC 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.
Common problems reported for Atto 8 →Atto 8 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.