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B164C

DTC B164C indicates an unintended electrical connection between the front passenger-side seat belt pretensioner control circuit and the vehicle power supply (B+) — Seal U

Safety System

DTC B164C indicates an unintended electrical connection between the front passenger-side seat belt pretensioner control circuit and the vehicle power supply (B+).

The seat belt pretensioner is a key actuator in the SRS system and contains a pyrotechnic device.

Under normal conditions, the airbag ECU triggers the pretensioner only during a collision via a low-level signal.

A short to power continuously exposes the pretensioner supply or control circuit to 12V.

This fails the system self-check, illuminates the SRS warning lamp, and creates a severe safety risk: the short-circuit current may accidentally trigger the pretensioner (instantly and permanently locking the seat belt), or circuit protection mechanisms may prevent proper pretensioner deployment during a collision.

Repair this hard short-circuit fault immediately to ensure full passive safety system functionality.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Mechanical wear of the under-seat wiring harness: Frequent fore/aft movement of the front passenger seat or pinching by objects damages the pretensioner wiring harness insulation, causing it to contact the seat frame or power wiring harness and create a short circuit.
  • 2Connector water ingress or corrosion: Spilled drinks, improper car washing, or wading causes the internal terminals of the pretensioner connector (usually located below the B-pillar or under the seat) to short circuit, or causes electrochemical migration after the metal terminals oxidize.
  • 3Pretensioner internal coil insulation failure: Aging, overheating, or manufacturing defects damage the insulation layer of the pretensioner internal squib coil, causing the coil to short to the housing (ground) or power supply line.
  • 4Improper repair work: Failure to reinstall wiring harness retaining clips during previous interior trim removal, seat repairs, or carpet cleaning caused the seat slide rail to crush the wiring harness or a retaining screw to pierce the insulation.
  • 5SRS ECU internal drive circuit fault: The pretensioner drive MOSFET or capacitor inside the airbag control module breaks down, causing the output terminal to continuously output supply voltage instead of the normal low-level control signal.
What To Do
  • 1
    Safe power-down and wait: Turn the power switch to OFF, disconnect the 12V battery negative terminal, and wait at least 90 seconds (to fully discharge the SRS capacitor and prevent accidental deployment).
  • 2
    Fault status confirmation: Reconnect the diagnostic tool, read the Freeze Frame data, confirm B164C is a current fault (Active) rather than a history fault, and record the vehicle status when the fault occurred.
  • 3
    Visual and physical inspection: Remove the front passenger seat (if necessary). Carefully inspect the wiring harness under the seat, at the bottom of the B-pillar, and the floor harness. Check the wiring harness for abrasion, cuts, or burn marks. Inspect connector KJ10 (or the corresponding model-specific connector) for water ingress, corrosion, terminal back-out, or foreign matter.
  • 4
    Circuit parameter measurement: Disconnect the front passenger seat belt pretensioner connector. Use a multimeter to measure the voltage from the wiring harness side terminal to ground (Normal: <1V. A reading of 12V confirms a short to power). Measure the resistance from the terminal to power (Normal: >1MΩ). Measure the pretensioner unit resistance (Normal: 2.0-3.0Ω. A reading of <1Ω or infinite indicates an internal short or open circuit).
  • 5
    Wiring harness insulation repair: If the wiring harness is damaged, apply heat-shrink tubing or wrap it in layers of insulating tape. Maintain a safe distance from the vehicle body metal and power cables. If the wiring harness is worn at the seat slide rail, reroute the harness and install a protective corrugated conduit.
  • 6
    Component replacement and verification: If the pretensioner is internally shorted, replace the front passenger seat belt pretensioner assembly (replace the seat belt assembly or pretensioner module simultaneously). If the fault persists after repairing the wiring harness, measure the output voltage at the SRS ECU terminal. If abnormal, replace the airbag control module.
  • 7
    System reset and self-check: Restore all connections and connect the battery. Set the power mode to ON (do not start). Wait 20 seconds for the SRS system to self-check and confirm the fault light turns off. Use the diagnostic tool to clear the fault code. Perform a collision simulation test (using a dedicated resistor substitute) to verify system function.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Qin Pro DM - Seat rail crushed wiring harness causing short to power

✓
The SRS warning light stayed on constantly when the vehicle arrived. The scan tool read DTC B164C-00 (current). Inspection found the wiring harness retaining clip under the front passenger seat had detached, allowing the harness to drop into the seat slide rail. Seat movement caused the rail edge to chafe through the insulation of the brown power wire (B/Y), which shorted to the pretensioner signal wire (Y/L). Removed the seat, repaired the harness with double-layer heat shrink tubing, rerouted the wiring, and installed a protective bracket. Cleared the fault code and the system returned to normal.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Tang EV - Connector corroded and short-circuited after water ingress

✓
One week after the water ingress repair, the SRS warning light came on. Diagnosis showed DTC B164C. Inspecting the pretensioner connector KJ10 under the passenger seat found obvious water stains and green copper corrosion inside the connector. Electrolyte residue between the terminals created a conductive path, shorting to power. Cleaned the connector terminals with electronic cleaner and anti-rust spray, blew out the moisture, applied conductive grease, and reconnected. Advised the owner to check the floor seals to prevent further water ingress.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Song MAX - Pretensioner Internal Short Circuit Replacement Case

✓
After a front-end collision repair (bumper and radiator support replaced), the SRS warning light stayed on. Diagnosis identified a faulty front impact sensor and DTC B164C. The voltage at the passenger pretensioner harness was normal (0V), but the pretensioner itself measured only 0.8Ω (spec: 2–3Ω). Determined that the pretensioner had been damaged in the collision: the impact damaged the internal igniter insulation, creating an internal short to power. Replacing the passenger seatbelt assembly (including pretensioner) resolved the fault.
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Yuan EV – Retrofitting heated seats causes circuit interference

✓
The owner installed aftermarket seat heating pads, cutting into the pretensioner power circuit to source power. This caused 12V constant power interference on the pretensioner control line, triggering DTC B164C. Inspection revealed non-factory wiring beneath the front passenger seat; the heating pad power cable had poorly wrapped insulation tape and was contacting the pretensioner signal wire. Removed the modified wiring and restored the original harness insulation. Advised the owner that SRS circuits must never be modified. Fault resolved.
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Qin Petrol - SRS ECU Internal Driver Circuit Fault

✓
After multiple repairs, the B164C fault kept returning. The wiring harness and pretensioner were both normal. In-depth inspection revealed burn marks on the pretensioner drive circuit board inside the airbag ECU (beneath the centre console). A MOSFET breakdown caused continuous high-level output. Replaced the airbag control module (required simultaneous VIN writing and configuration code matching), completed online programming, and permanently resolved 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]