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B177C

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

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 Qin Plus 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 from deploying correctly and trigger the airbag warning light and buzzer. Common causes include worn under-seat wiring or moisture/corrosion in 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 — Qin Plus
  • 1Under-seat wiring harness mechanical wear: Frequent sliding or folding of the left middle-row seat wears through the pretensioner wiring harness insulation (usually located near the seat rail). Contact with the metal vehicle frame creates 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 unit internal short circuit: 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 the body ground.
  • 5Seat modification damage: Retaining screws pierce the pretensioner wiring harness, causing a short to ground during customer installation of seat covers, heating pads, or electric adjustment mechanisms.
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 directly measure the pretensioner connector (use a dedicated SRS resistance measuring tool or the diagnostic tool active test function).
  • 2
    Visual inspection: Inspect the wiring harness sleeve 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) and use the diagnostic tool to read the fault code status. If the status changes to an open circuit fault (B177D), the short circuit is in the pretensioner assembly. 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. Normal reading is infinity. If continuous, open the corrugated conduit section by section along the harness to check for damage, focusing on the seat hinge movement area.
  • 5
    Pretensioner resistance measurement: Use the dedicated SRS test resistor (2Ω substitute resistor) or the scan tool actuator test function to verify the pretensioner unit resistance is within the standard range (1.5-3.0Ω). If out of range, replace the pretensioner assembly.
  • 6
    Insulation repair: Upon finding harness damage, 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 wiring harness sleeve or securing cable ties if necessary.
  • 7
    Connector handling: If water is present in the plug, 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 access the SRS system and execute 'Configuration and Settings' → 'Seat Belt Pretensioner Check'. Clear the fault code and perform an ignition circuit self-check. Verify the fault code changes to 'History Fault' and the warning lamp 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 trigger actual deployment). Confirm 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 Qin Plus 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.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.B110BDTC B110B indicates a fault in the internal air pump (sampling pump) of the PM2.5 air quality detector. This pump draws cabin or outside air into the PM2.5 sensor detection chamber, enabling the laser scattering sensor to measure real-time particulate matter concentration. The air conditioning controller (or BCM) sets this DTC upon detecting an open or short circuit in the pump drive circuit, or abnormal current caused by a seized pump motor. This fault disables the "Green Clean" system, causes the instrument cluster air quality display to read abnormally or disappear, and disables the automatic fresh air/recirculation switching function. It does not affect the vehicle powertrain or core thermal management functions.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.B173BDTC B173B indicates an abnormal electric A/C compressor speed feedback signal or a communication fault between the compressor controller and the motor. In BYD new energy models (e.g., Tang, Song, Qin, and Yuan series), this DTC belongs to the HVAC subsystem within the Body Control system and specifically involves the electric compressor closed-loop control circuit. The ECU sets this DTC when the compressor controller (IPM) fails to receive a valid pulse signal from the Hall speed sensor for more than 3 continuous seconds, or when the deviation between the detected actual speed and the target commanded speed exceeds the calibrated threshold (typically >500 rpm). This fault forces the A/C system into fail-safe mode and stops compressor operation. Symptoms include a complete lack of cooling, intermittent cooling, or a sharp decrease in cooling performance. The instrument cluster A/C warning lamp may also illuminate. Prolonged operation with this fault can degrade high-voltage system insulation or cause controller overcurrent damage.
Common problems reported for Qin Plus →Qin Plus 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.