AU
B177D1A

DTC B177D1A indicates the SRS (Supplemental Restraint System) control unit detects the right middle-row seat belt pretensioner circuit resistance is abnormally low, approaching 0 ohms (standard value is typically 2 — Atto 8

Safety System

DTC B177D1A indicates the SRS (Supplemental Restraint System) control unit detects the right middle-row seat belt pretensioner circuit resistance is abnormally low, approaching 0 ohms (standard value is typically 2.0–5.0 Ω).

This indicates a short to ground, a short between wires, or an internal short circuit in the pretensioner squib circuit.

This fault causes the SRS to enter a degraded protection mode.

During a collision, the right middle-row seat belt pretensioner may fail to deploy and retract.

The fault also illuminates the Airbag Warning Light continuously, compromising occupant passive safety protection.

3
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Damaged pretensioner wiring harness insulation under the right middle-row seat or near the B-pillar causes the wire to contact metal body components, creating a short to ground.
  • 2Water ingress, moisture, or corrosion at the seat belt pretensioner connector (usually located under the seat rail or inside the B-pillar trim), causing a short circuit between terminals or a short to ground.
  • 3A manufacturing defect or aging causes an internal short circuit in the pretensioner squib, dropping the resistance to 0 Ω.
  • 4Water enters the SRS wiring harness connector during vehicle wading or interior cleaning, causing a temporary or continuous short circuit.
  • 5Frequent fore/aft seat adjustment or seat modification causes mechanical interference between the wiring harness and seat frame. Prolonged friction damages the wiring harness and causes a short circuit.
What To Do
  • 1
    Perform high-voltage safety procedures: Disconnect the low-voltage battery negative terminal and wait at least 3 minutes to fully discharge the SRS backup power supply and prevent accidental airbag deployment.
  • 2
    Locate the right middle row seat belt pretensioner: Remove the right middle row seat (or check below the B-pillar) and find the pretensioner assembly and its 2-pin connector (usually a yellow waterproof connector).
  • 3
    Visual inspection: Check the connector for signs of water ingress, terminal corrosion, burn marks, or physical damage. Check the wiring harness for wear, pinching, or breakage.
  • 4
    Resistance measurement (disconnected state): Use a multimeter to measure the resistance between the pretensioner body terminals. Normal resistance is 2.0-5.0Ω; if it measures 0Ω, replace the pretensioner assembly.
  • 5
    Harness continuity check: Measure resistance to ground at the harness-side connector (should be infinite). Measure continuity between both ends of the harness. If continuity to ground exists, inspect the harness section by section to locate the damaged point.
  • 6
    Insulation repair or replacement: If the wiring harness shows damage, repair it using heat-shrink tubing or waterproof tape; if the connector shows damage, replace the connector; if the wiring harness shows extensive damage, replace the entire SRS wiring harness.
  • 7
    System reset and verification: Reconnect all connectors and the battery. Use VDS or a dedicated diagnostic tool to clear the fault code. Perform the SRS system self-check procedure. Confirm B177D1A does not return and the airbag warning light is off.
Real-World Cases
BYD DTC AI Analysis

Water-damaged vehicle: Right middle row pretensioner circuit short

✓
Symptoms: A 2020 BYD E3 developed a constant airbag warning light after wading through deep water in heavy rain. VDS read DTC B177D1A. Diagnosis: Removed the right second-row seat. Found obvious water stains in the SRS harness connector beneath the seat, with terminals oxidized. Measured 0.2 ohms between the pretensioner harness side and ground, confirming a short to ground. Harness insulation had worn through at the seat rail mounting clip; water entered during wading and caused the short. Repair: Cleaned the connector terminals with anhydrous alcohol, thoroughly dried with a heat gun, repaired the damaged harness insulation, rewrapped with waterproof tape, and rerouted the harness to avoid contact with the seat frame. Cleared the fault codes; system returned to normal.
BYD DTC AI Analysis

Seat modification damaged the pretensioner wiring harness

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Symptoms: The 2019 Qin EV displayed DTC B177D1A and the airbag warning light illuminated after the owner installed a seat cover on the right-hand middle row seat. Diagnosis: Inspection revealed the seat cover mounting clip interfered with the pretensioner wiring harness inside the seat backrest. After removing the seat, we found the clip screw had pierced the harness, causing both positive and negative wires to contact the seat metal frame and short. We measured the pretensioner resistance at 3.2Ω (normal), but the harness side showed 0Ω to ground. Solution: We repaired the damaged harness, replaced the broken wires, rerouted the wiring with corrugated tubing protection, repositioned the seat cover mount to clear the harness routing, and cleared the fault.
BYD DTC AI Analysis

Water ingress corroded the connector, causing an intermittent short circuit

✓
Symptoms: The 2019 E2 airbag warning light came on intermittently during the rainy season, with DTC B177D1A appearing sporadically. Diagnosis: The right middle row seatbelt pretensioner connector seal had degraded, with green corrosion on the terminals. Resistance read 0Ω in damp conditions but returned to normal when dry. Moisture entered through the failed seal, causing a short circuit at specific humidity levels. Repair: Installed a new waterproof connector (complete with seal and terminals) and waterproofed the wiring harness. Fault eliminated.
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.