AU
B164C-00

This DTC indicates an abnormal short circuit between the front passenger seat belt pretensioner ignition circuit and the vehicle power supply (+B, typically the positive terminal of the 12V battery) — Seal U

Safety System

This DTC indicates an abnormal short circuit between the front passenger seat belt pretensioner ignition circuit and the vehicle power supply (+B, typically the positive terminal of the 12V battery).

The pretensioner is a pyrotechnic device.

During a collision, the SRS control unit sends an ignition signal to trigger the internal gas generator and rapidly tighten the seat belt.

A short to power in the pretensioner circuit indicates damaged wiring insulation or internal connector bridging, applying a continuous high potential across the pretensioner terminals.

This condition forces the SRS system into fail-safe mode, disabling the front passenger airbag and pretensioner functions.

Extreme cases risk unintended pretensioner deployment or complete failure, posing a severe safety hazard.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Chafed or crushed wiring harness under the front passenger seat: Seat fore-and-aft adjustment or passenger foot movement damages the pretensioner wiring harness insulation (usually located near the seat rail), causing it to contact power wires (such as seat heating or seat adjustment motor power wires) and short circuit.
  • 2Water ingress at the B-pillar or sill wiring harness connector: Vehicle wading or poor sealing corrodes the internal terminals of the pretensioner connector (usually located behind the B-pillar trim panel or under the seat), creating a conductive path that causes a short to power.
  • 3Pretensioner internal fault: Insulation failure of the igniter (squib) inside the pretensioner assembly shorts the ignition terminal to the housing or power terminal. This condition typically causes abnormal resistance (below 1.5 Ω or above the normal range).
  • 4SRS control unit internal driver circuit fault: An internal ignition drive transistor breakdown or filter capacitor short circuit causes the pretensioner terminal to output a continuous high level.
  • 5Non-professional modifications: Piercing the pretensioner wiring harness during improper installation of seat ventilation, heating, or ambient lighting, or incorrectly splicing the pretensioner circuit into a constant power circuit.
What To Do
  • 1
    Safety preparation: Disconnect the 12V battery negative terminal and wait at least 90 seconds to fully discharge the SRS capacitor and prevent accidental airbag deployment.
  • 2
    Fault confirmation: Use a dedicated BYD diagnostic tool (VDS or ED400) to read fault codes. Confirm B164C-00 is a current fault (Active) and record freeze frame data. Check for accompanying B164B (open circuit) or B164D (short to ground) fault codes.
  • 3
    Visual inspection: Remove the front passenger seat (if applicable) and the lower B-pillar trim panel. Inspect the pretensioner connector (typically a yellow 2-pin plug) for water ingress, burn marks, bent pins, or corrosion. Inspect the wiring harness under the seat for wear, damaged insulation, or interference with the seat track.
  • 4
    Circuit measurement: Disconnect the SRS control unit and pretensioner connectors. Use a multimeter to measure the voltage between the pretensioner harness side and the power supply (should be <1V). Measure the pretensioner resistance (standard value: 2.0-3.0Ω, varies slightly by model). Check the harness for a short to power (continuity between the pretensioner connector terminal and the body power supply should be infinite).
  • 5
    Wiring harness repair: If the wiring harness is damaged, wrap it with high-temperature insulating tape (≥125°C) or replace the wiring harness assembly. If water has entered the connector, clean it with electrical contact cleaner, blow it dry, and replace the connector if necessary. Fully seat the wiring harness retaining clips to prevent interference with moving parts.
  • 6
    Component replacement: If the pretensioner has abnormal resistance (<1Ω or >5Ω) or an internal short to power, replace the front passenger seat belt pretensioner assembly. (Note: The pretensioner is a single-use component. Replace it if dropped or subjected to impact.)
  • 7
    System reset: Reconnect all connectors, restore battery power, and use the diagnostic tool to clear the fault code. Perform the SRS system self-test (typically requires turning the ignition switch to the ON position and verifying the SRS warning lamp turns off). Finally, perform a dynamic test (simulate a crash signal using special equipment; do not perform an actual vehicle crash test).
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Seat rail wear caused pretensioner short circuit in Qin EV450

✓
A 2018 Qin EV450 with 32,000 km had the SRS warning light staying on. Scanning showed DTC B164C-00. The wiring harness retaining clip under the passenger seat had come loose, letting the harness chafe against the metal edge of the seat track. The insulation wore through and the exposed copper wires contacted the seat heater constant +12V feed. Repaired the two damaged wires (pretensioner trigger and return), re-secured the harness, and added corrugated conduit for protection. Fault resolved.
BYD DTC AI AnalysisFrom Chinese market (translated)

B-Pillar Connector Corrosion Case: Water-Wading Vehicle

✓
2017 BYD Qin 100. Dashboard displayed airbag fault after driving through floodwater during a typhoon. Diagnostic scan found codes B164C-00 and B1650-00 (driver's side equivalent). Removed the passenger side B-pillar trim and found visible water staining and green corrosion inside the yellow pretensioner connector. Degraded insulation resistance between terminals caused a short to power. Cleaned the connector with anhydrous alcohol, dried it, applied conductive protective grease, and replaced the waterproof seal. Fault resolved. Check the sunroof drain tubes and door seals.
BYD DTC AI AnalysisFrom Chinese market (translated)

Internal short in pretensioner after accident repair

✓
After the Qin EV300’s front collision repair, the SRS light stayed on. The front passenger airbag had been replaced after the impact, but the seatbelt was not. Diagnostics found DTC B164C-00. The pretensioner measured 0.8 Ω (too low) and was shorted to power. Disassembly revealed that the collision impact caused a breakdown short circuit in the semiconductor bridge (SBB) inside the pretensioner’s igniter. Replacing the front passenger seatbelt assembly (including pretensioner) cleared the fault codes and restored normal operation.
BYD DTC AI AnalysisFrom Chinese market (translated)

Aftermarket heated seat modification caused a fault.

✓
The owner installed an aftermarket ventilation and heating pad on the passenger seat. During installation, they overtightened the cable ties securing the wiring, which cut into and damaged the factory pretensioner wiring harness. They also incorrectly tapped the heating pad into the pretensioner circuit for power. This triggered DTC B164C-00 and heating function faults. Repair involved removing the modified wiring, repairing the original harness (two damaged wires), restoring the factory circuit layout, and advising the owner that airbag system wiring must never be modified.
BYD DTC AI AnalysisFrom Chinese market (translated)

SRS control unit internal fault misdiagnosis case

✓
A BYD Qin 80 repeatedly set DTC B164C-00. Previous repairs replaced the pretensioner and wiring harness, but the fault recurred intermittently. The root cause turned out to be a thermally unstable firing driver chip (typically a high-power MOSFET) inside the SRS control unit. After a period of operation, the chip broke down and shorted to power. Replacing the SRS control unit (under the center console or inside the dashboard) and performing vehicle configuration programming permanently fixed the fault.
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]