AU
B177E

DTC B177E indicates the SRS (Supplemental Restraint System) detects the resistance of the right middle-row seat belt pretensioner (typically the second-row right seat belt retractor pretensioner) falls below the system threshold (generally below 1 — Seal U

Safety System

DTC B177E indicates the SRS (Supplemental Restraint System) detects the resistance of the right middle-row seat belt pretensioner (typically the second-row right seat belt retractor pretensioner) falls below the system threshold (generally below 1.0-1.5Ω, depending on vehicle calibration).

As a pyrotechnic actuator in the airbag system, the pretensioner contains an internal resistance heating wire.

Standard resistance typically falls within 2.0±0.5Ω.

Low resistance indicates a short circuit risk in the loop.

Potential causes include an inter-turn short in the internal pretensioner coil, a wiring harness short to ground, or a short between connector terminals.

This fault causes the SRS ECU to register abnormal pretensioner operation.

During a collision, the ECU may inhibit pretensioner deployment, preventing the seat belt from retracting.

The fault also continuously illuminates the airbag warning lamp and triggers a buzzer alarm on certain models.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Pretensioner internal short circuit: Damaged or damp insulation on the pretensioner internal heating coil causes an abnormal drop in resistance (measured value may be <1.0Ω).
  • 2Harness short to ground: Long-term friction or compression damaged the insulation on the wiring harness under the right middle-row seat or the floor harness, grounding it to the vehicle body metal.
  • 3Connector water ingress and corrosion: Vehicle wading, water entering the floor, or water flowing into the pretensioner connector (usually located under the seat or below the B-pillar) during vehicle washing causes a short circuit between terminals or a short to ground.
  • 4SRS ECU detection error: Airbag control module internal sampling circuit fault or reference resistor drift causes a false low resistance reading.
  • 5Improper repair procedures: Measuring the pretensioner with a low-impedance multimeter while energized, or pinching the wiring harness with the seat rail during previous repairs.
What To Do
  • 1
    Safety preparation: Disconnect the 12V battery negative terminal and wait at least 90 seconds (3 minutes for some models) to fully discharge the SRS capacitor and prevent accidental airbag deployment.
  • 2
    Physical inspection: Check the right second-row seat belt assembly appearance, the wiring harness routing under the seat, and the pretensioner connector (usually located at the seat base or inside the side trim panel) for water stains, corrosion, damage, or backed-out terminals.
  • 3
    Pretensioner unit measurement: Disconnect the pretensioner connector. Measure the resistance between the two pretensioner terminals using a high-precision digital multimeter (0.1Ω resolution). Standard value: 1.5-3.0Ω. If <1.0Ω, replace the pretensioner assembly.
  • 4
    Harness insulation check: Measure the resistance between the pretensioner connector (harness side) and body ground (should be >1MΩ). Measure the resistance between the two harness wires (should be >1MΩ). Locate the short circuit.
  • 5
    ECU-side inspection: Check the corresponding SRS ECU connector terminals (refer to the circuit diagram; usually pins 19/20 or marked RMPR/RMPR+) for oxidation or deformation. Measure the wiring harness continuity between the ECU and the pretensioner.
  • 6
    Replacement verification: Connect a standard 2.0Ω resistor or a known-good pretensioner to the circuit. Clear the fault code and observe if it returns to determine whether the fault is in the wiring or the ECU.
  • 7
    System reset: After confirming fault resolution, connect the battery, clear the fault code using a VDS or X-431 diagnostic tool, perform an SRS system self-test, and verify normal quiescent current and communication.
Real-World Cases
BYD DTC AI Analysis

Song MAX right second-row pretensioner short circuit after water wading

✓
Vehicle wading depth exceeded floor level; SRS warning light stays on. Retrieved DTC B177E. Removed right middle row seat and found water stains on pretensioner connector (inner side of seat rail). Corroded terminals caused a short between positive and negative, measuring 0.6Ω. Cleaned connector with electronic cleaner, dried with compressed air; measured pretensioner body resistance at 2.1Ω (normal). Re-wrapped connector with waterproof tape and adjusted harness position to avoid water pooling in low spots. Fault resolved.
Original source ↗
BYD DTC AI Analysis

Qin Pro EV pretensioner resistance fault after accident repairs

✓
Right rear impact repair. Cluster showed "Check Airbag System." Scan tool retrieved DTC B177E. Right middle row seat belt pretensioner measured 0.8Ω (below specification)—the collision had internally damaged the pretensioner and partially shorted the coil. Replaced the right middle row seat belt assembly (includes pretensioner). Performed configuration coding with the diagnostic tool (programmed VIN and vehicle configuration). Cleared fault codes; system normal.
Original source ↗
BYD DTC AI Analysis

Tang DM Intermittent B177E Troubleshooting

✓
SRS warning light occasionally lit on rough roads; remained off on smooth surfaces. Retrieved historical DTC B177E. Found the right middle row seatbelt pretensioner harness retaining clip loose behind the seat rail. Seat movement rubbed the harness against the rail edge, damaging insulation and causing intermittent short to ground. Repaired the damaged wiring, wrapped it in abrasion-resistant corrugated tubing, and re-secured the routing to clear moving components. Fault has not recurred.
Original source ↗
BYD DTC AI Analysis

Poor contact at e5 pre-tensioner connector caused abnormal resistance.

✓
SRS light stayed on. Scan showed DTC B177E. Found the right middle row seatbelt pretensioner connector not fully locked (locking tab fractured), causing poor terminal contact and abnormal resistance (resistance intermittently read low). Also found slight oxidation inside the connector. Replaced the connector housing, cleaned the terminals, applied conductive protectant, and secured the connection. Circuit resistance measured 2.0Ω. Cleared the fault code; issue resolved.
Original source ↗
BYD DTC AI Analysis

Aftermarket audio installation on Song DM accidentally damaged seatbelt pretensioner wiring harness

✓
The SRS warning light illuminated after the owner installed floor audio equipment. Inspection revealed that during the modification, the right-hand second-row seat mounting bolt pinched the seatbelt pretensioner harness, fracturing the internal copper conductors and shorting them to ground. Resistance measured 0.3Ω. Stripping back the harness showed both wire insulations were damaged. Cut out the damaged section (leaving sufficient length), re-soldered the wires using waterproof heat-shrink tubing, and re-routed the harness to clear the seat mounting points. Fault resolved.
Original source ↗
Other Seal U 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.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.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.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.
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.