AU
B164C

DTC B164C indicates an unintended electrical connection between the front passenger-side seat belt pretensioner control circuit and the vehicle power supply (B+) — Atto 3

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 — Atto 3
  • 1Mechanical wear of the under-seat wiring harness: Frequent front passenger seat movement or pinching by objects damages the pretensioner wiring harness insulation, causing a short circuit to the seat frame or power wiring harness.
  • 2Connector water ingress or corrosion: Spilled drinks, improper car washing, or wading causes a short circuit across the internal terminals of the pretensioner connector (usually located below the B-pillar or under the seat), or metal terminal oxidation causes electrochemical migration.
  • 3Pretensioner internal coil insulation failure: Aging, overheating, or manufacturing defects damage the pretensioner internal squib coil insulation layer, causing the coil to short to the housing (ground) or power supply wire.
  • 4Improper repair work: Failure to reinstall wiring harness retaining clips during previous interior trim removal and installation, 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 shorts, 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 and read the Freeze Frame data. Confirm B164C is a current fault (Active) rather than a history fault. Record the vehicle status when the fault occurred.
  • 3
    Visual and physical inspection: Remove the front passenger seat (if necessary) and carefully inspect the wiring harness under the seat, at the base of the B-pillar, and along the floor. Check the wiring harness for wear, cuts, or burn marks. Check connector KJ10 (or the corresponding vehicle model connector) for water ingress, corrosion, terminal back-out, or foreign material.
  • 4
    Circuit parameter measurement: Disconnect the front passenger seat belt pretensioner connector. Use a multimeter to measure the voltage from the harness-side terminal to ground (normal: <1V; a 12V reading confirms a short to power). Measure the resistance from the terminal to power (normal: >1MΩ). Measure the pretensioner component resistance (normal: 2.0-3.0Ω; a reading <1Ω or infinite indicates an internal short or open circuit).
  • 5
    Wiring harness insulation repair: If the wiring harness is damaged, wrap it in layers using heat-shrink tubing or insulating tape. Maintain a safe distance from 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 (simultaneously replace the seat belt assembly or pretensioner module). 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, connect the battery, and set the power mode to ON (do not start). Wait 20 seconds for the SRS system to self-check and confirm the fault indicator turns off. Use the diagnostic tool to clear the fault code. Perform a crash 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 Atto 3 Fault Codes
P0A0DBattery Energy Control Module Requested MIL IlluminationP0AA6Hybrid Battery Voltage System Isolation FaultP1A00Drive Motor "A" Performance — BYD proprietaryB1108For 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.
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]