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B1677

DTC B1677 indicates a short to ground in the left rear side impact sensor (SIS) signal circuit — Seal U

Safety System

DTC B1677 indicates a short to ground in the left rear side impact sensor (SIS) signal circuit.

In the BYD SRS (Supplemental Restraint System) architecture, the left rear side impact sensor typically mounts below the B-pillar or near the left rear door frame to monitor collision acceleration at the left rear of the vehicle.

When the sensor signal wire shorts to the vehicle body ground, the SRS control module (ACM) detects a signal voltage continuously below the threshold (usually <0.5V) and logs a short-to-ground fault.

This fault disables the left rear side collision detection function, forces the system into fail-safe mode, and illuminates the instrument cluster SRS warning lamp.

During a side collision, the left-side curtain airbag and seat side airbag may fail to deploy, posing a serious safety risk.

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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Internal short circuit in the sensor body: Damage to the piezoelectric element or signal processing circuit inside the left rear side impact sensor causes the resistance between the signal terminal and ground terminal to drop abnormally (<10Ω).
  • 2Harness insulation worn and shorted to ground: Vibration and friction damage the insulation where the harness passes through body sheet metal holes, harness retaining clips, or floor routing channels, causing the copper core to contact the vehicle body metal.
  • 3Connector water ingress and corrosion: Vehicle wading, car washing, or poor sealing allows water to enter the sensor connector (usually located inside the B-pillar trim or under the seat), forming an electrolytic conductive path between the pins.
  • 4Improper repair causing wiring harness compression: Seat rails, seat belt anchors, or trim panel clips crush and damage an incorrectly secured wiring harness after accident repairs or interior trim removal and installation.
  • 5SRS control module internal fault: ACM internal signal acquisition circuit short to ground (low probability; confirm only after ruling out wiring harness and sensor).
What To Do
  • 1
    Safety Preparation: Disconnect the 12V battery negative terminal. Wait at least 90 seconds to discharge the SRS system capacitor and prevent accidental airbag deployment.
  • 2
    Fault confirmation: Use a BYD VDS or Launch X-431 diagnostic tool to read the fault code. Confirm B1677 is a current fault (Active) and record freeze frame data (vehicle speed, timestamp, etc.).
  • 3
    Visual inspection: Remove the left B-pillar lower trim panel and rear door sill trim panel. Inspect the crash sensor for physical damage, water stains, or burn marks. Inspect the wiring harness for obvious wear, damaged insulation, or crushing.
  • 4
    Electrical measurement: Disconnect the sensor connector and use a multimeter to measure the resistance between the wiring harness side signal pin (usually connector pin 2) and body ground. Normal resistance is >1 MΩ. If the resistance is <10 Ω, the wiring harness is shorted to ground; trace the circuit to locate the fault.
  • 5
    Sensor inspection: Measure the resistance between the sensor body signal terminal and the ground terminal. The normal value is 2-5kΩ (refer to the vehicle repair manual for specific values). If the resistance is close to 0Ω, replace the sensor.
  • 6
    Wiring harness repair: If damaged, repair the wiring harness using waterproof insulating tape. Replace the wiring harness assembly if necessary. Check for missing wiring harness retaining clips. Ensure the wiring harness maintains a safe distance from sharp metal edges.
  • 7
    Connector handling: If the connector has water ingress, clean the pins with electrical contact cleaner, apply conductive grease, and replace the sealing ring or the entire connector.
  • 8
    Component replacement: If testing confirms a sensor fault, replace the left rear side impact sensor (verify the part number by vehicle model, as Yuan EV and Tang parts may differ). Typical installation torque is 8-10 N·m. Verify correct sensor orientation (arrow pointing toward the front of the vehicle).
  • 9
    System calibration: Reconnect the battery, use the diagnostic tool to perform the 'SRS System Self-Learning' or 'Crash Sensor Configuration' procedure, and clear the fault code.
  • 10
    Function check: Perform an ignition cycle test and confirm the instrument panel SRS warning lamp turns off normally. Use the diagnostic tool to read the data stream and verify the left rear side impact sensor acceleration value is within 0±0.5G. Perform a simulated impact test (using the dedicated test tool) to verify system response.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Song MAX B1677 fault after wading

✓
A 2019 BYD Song MAX owner reported that the airbag warning light stayed on after driving through water. The technician used the VDS to read fault codes and found DTC B1677 (active). Removing and inspecting the left rear side impact sensor (located below the left C-pillar) revealed water stains and green copper corrosion inside the connector. The water level had exceeded the door sill seal during the crossing, allowing water to enter the connector through the wiring harness waterproof boot. The technician cleaned the connector pins with anhydrous alcohol, applied silicone-based conductive grease, replaced the waterproof sealing ring, and re-secured the wiring harness waterproof boot. After clearing the fault code, the SRS light went out and the data stream showed the sensor voltage returned to normal (2.3V).
BYD DTC AI AnalysisFrom Chinese market (translated)

Tang DM wiring harness chafing after accident repair

✓
2021 Tang DM. DTC B1677 appeared one month after left rear accident repair. The inspection found mechanics had replaced the left rear wing during the accident repair, but had not installed the wiring harness retaining clips to standard. This allowed the impact sensor wiring harness (B-pillar to floor section) to chafe against the metal edge of the seat rail. Damaged insulation caused a short to earth. Repair: Repaired the damaged harness (3 wires, including the signal wire shorted to earth). Wrapped the harness in corrugated tubing, re-routed and secured it to specification. Fitted a rubber grommet to isolate the metal edge. Replaced the impact sensor (the short circuit may have damaged the internal protection diode).
BYD DTC AI AnalysisFrom Chinese market (translated)

Internal short circuit in Yuan EV sensor

✓
2019 BYD Yuan EV535, 80,000 km. Intermittent B1677 fault code (alternating between current and historic). Fault not present during inspection. Measured harness-to-ground resistance: normal. Monitored sensor signal line with oscilloscope; detected occasional 0V pull-down. Diagnosed as intermittent short-to-ground inside the sensor (MEMS chip or filter capacitor). Replaced left rear side impact sensor (Part No.: BYD-3636400-XXX). When installing, make sure the arrow mark on the sensor points toward the front of the vehicle. After replacement, monitored continuously for one week — fault did not reoccur.
BYD DTC AI AnalysisFrom Chinese market (translated)

Seatbelt pretensioner incorrectly connected on Qin Pro, causing short circuit

✓
2020 BYD Qin (petrol). When retrofitting rear seat heaters, the modification shop mistakenly connected the left rear side impact sensor connector (white 2-pin) to the power tap wire near the seatbelt pretensioner connector (yellow 2-pin). This shorted the sensor power wire to ground, triggering DTC B1677, accompanied by DTC B1680 (seatbelt pretensioner fault). Resolution: Removed the improperly modified wiring harness, repaired the factory wiring harness insulation, and restored the original vehicle circuit. Reset the SRS control module coding, cleared the fault codes, and the system returned to normal.
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. Sources: [1]