AU
B164C12

DTC B164C12 indicates the airbag control unit (SRS ECU) detects an abnormally low-impedance connection between the front passenger seat belt pretensioner drive circuit and the vehicle power supply (B+) — Atto 3

Safety System

DTC B164C12 indicates the airbag control unit (SRS ECU) detects an abnormally low-impedance connection between the front passenger seat belt pretensioner drive circuit and the vehicle power supply (B+).

The pretensioner is essentially an electric squib with a normal operating resistance of approximately 2.0-3.0Ω.

When damaged wiring harness insulation causes contact with a constant power wire, the ECU detects an abnormal voltage (approaching 12V/B+ voltage) instead of the expected open circuit or low-resistance state.

This fault triggers the SRS fail-safe mode.

The system disables the front passenger airbag and pretensioner functions to prevent injury from accidental deployment, and illuminates the airbag warning light to alert the driver.

Excessive short-circuit current also risks unintended pretensioner deployment or burning out the SRS driver chip.

5
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Wiring harness abrasion under the front passenger seat: Frequent forward and backward seat adjustment wears the insulation of the pretensioner wiring harness (usually routed near the seat rail), causing it to contact the seat heating power wire or constant power harness and create a short circuit.
  • 2Connector terminal displacement or water ingress: Water exposure, improper cleaning, or vibration causes terminal bridging in the pretensioner connector (yellow plug) located under the seat base or lower B-pillar; or a failed locking tab causes the terminal to contact the metal frame.
  • 3Internal short circuit in the pretensioner unit: Although rare, insulation breakdown in the pretensioner igniter can cause a short circuit between the terminals, to the housing (ground), or to the power supply wire.
  • 4Interference from modified or added accessory wiring: Unauthorized installation of seat heating pads, ventilation devices, or ambient lighting causes improper wire routing that interferes with the SRS wiring harness, or involves improper wire tapping methods.
  • 5SRS control unit internal driver chip fault: A damaged MOSFET or detection circuit in the ECU internal ignition driver circuit causes a false short-to-power fault (confirm via swap test).
What To Do
  • 1
    Safety preparation: Turn the vehicle OFF, disconnect the 12V battery negative terminal, and wait at least 90 seconds to fully discharge the SRS capacitor. Wear an anti-static wrist strap. Never directly measure the pretensioner using a multimeter resistance setting.
  • 2
    Visual inspection: Remove the front passenger seat (leave the wiring harness connected). Inspect the pretensioner wiring harness (yellow corrugated conduit) under the seat rail for abrasion or crushing marks. Focus on the crossover points with the seat heating module harness and the seat belt reminder sensor harness.
  • 3
    Connector inspection: Disconnect the pretensioner connector (yellow) under the seat. Inspect the terminals for corrosion, backed-out pins, metal burrs, or liquid ingress. Measure voltage from the connector harness side to ground and to B+ (reading must be 0V; if 12V, locate the short circuit).
  • 4
    Circuit continuity test: Use a multimeter to measure the insulation resistance between the pretensioner wiring harness (from the seat connector to the SRS ECU plug) and vehicle power lines (e.g., constant power, IGN power). The resistance must be greater than 10MΩ. If the resistance is low, inspect the harness in sections to locate the damage.
  • 5
    Pretensioner unit test: Measure the pretensioner resistance using a dedicated SRS tester (or the alternative 2Ω series resistor method). Normal range: 1.8-3.2Ω. If the resistance is abnormal or indicates an internal short circuit, replace the seat belt assembly (do not replace the pretensioner separately).
  • 6
    Repair and replacement: Repair the damaged wiring harness (use double-layer heat-shrink tubing for insulation and maintain the twisted-pair structure). If the wiring harness is severely damaged, replace the entire SRS wiring harness. When replacing a faulty pretensioner, verify the new part number matches (note actuator differences between ICE and electric vehicles).
  • 7
    System verification: Restore all connections and reconnect the battery. Use VDS or a dedicated diagnostic tool to clear fault codes. Perform an SRS system self-check (with the ignition switch ON, verify the warning light turns off). Perform a full-travel seat fore-and-aft adjustment test to confirm the fault does not recur.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin Pro DM seat rail crushed through the wiring harness, causing a short circuit.

✓
A 2019 BYD Qin Pro DM with 32,000 km came in with the SRS warning light on and DTC B164C12 stored. Inspection found that with the front passenger seat in its rearmost position, the seat rail mounting bolts pressed through the wiring harness corrugated tubing, causing the pretensioner power wire to contact the seat heating power wire (always live). The repair involved rerouting the harness, installing rubber protective sleeves for isolation, and repairing the damaged wires. Fault cleared.
BYD DTC AI AnalysisFrom Chinese market (translated)

E2 BEV: Connector corrosion and short circuit after wading through water

✓
2020 BYD E2. A blocked sunroof drain caused water to accumulate on the passenger side carpet, which spread to the SRS connector beneath the seat. Removed the pretensioner connector and found electrolytic corrosion between the terminals, creating a conductive path between the power and signal terminals. Thoroughly cleaned the connector, flushed it with electronic cleaner, dried it, applied conductive protectant, and replaced the damaged wiring harness terminals.
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin EV aftermarket seat heater installation causing interference

✓
2019 BYD Qin EV set DTC B164C12 one week after an aftermarket shop installed seat heaters. Inspection found the installer had spliced the heating pad power wire into the cigarette lighter power circuit and routed the wiring along the factory SRS harness. Metal harness clips had pierced the pre-tensioner harness insulation. Repair: Removed the non-compliant aftermarket wiring, re-wired the seat heaters following the factory routing, repaired the SRS harness insulation, and restored the system.
BYD DTC AI AnalysisFrom Chinese market (translated)

E3 pretensioner internal short circuit - replace assembly

✓
2020 E3 was involved in a minor rear-end collision (no airbag deployment). The SRS warning light then stayed on with DTC B164C12. Wiring harness inspection showed no faults. Pretensioner resistance measured 0.8 Ω with continuity to ground. Diagnosed as an insulation breakdown of the internal igniter in the pretensioner. Fix: Replaced the front passenger seatbelt assembly (including pretensioner), performed configuration coding and matching via VDS, and carried out crash sensor calibration.
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Qin Pro petrol — SRS ECU false fault

✓
2018 Qin Pro petrol variant intermittently sets DTC B164C12. Inspected wiring harness: normal. Measured pretensioner resistance: normal. Monitored the ECU driver output with an oscilloscope and detected abnormal voltage spikes. Confirmed poor thermal stability of the internal driver chip in the SRS ECU. Solution: Replaced the airbag control unit, rewrote the vehicle VIN and configuration parameters, and performed system calibration.
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]