B16C4
DTC B16C4 indicates an internal fault or severe communication failure in the airbag control unit (SRS ECU) — Atto 3
Safety System
DTC B16C4 indicates an internal fault or severe communication failure in the airbag control unit (SRS ECU).
This fault indicates a functional failure of the microprocessor, memory, or power management module within the SRS ECU, or a communication interruption between the ECU and the vehicle CAN network or sensor array.
The SRS ECU serves as the core controller of the passive safety system.
It receives crash sensor signals, determines collision severity, and triggers protective devices such as airbags and seat belt pretensioners.
This critical fault compromises occupant safety by potentially preventing normal airbag deployment during a collision, or by creating a risk of false alarms and unintended deployment.
When this fault occurs, the SRS system typically enters fail-safe mode, cuts off the airbag ignition circuit to prevent unintended deployment, and illuminates the instrument cluster airbag warning light.
The B16C4 fault code on the Atto 3 indicates an internal fault or severe communication failure in the airbag control unit. Severity is medium because the SRS system may enter fail-safe mode and airbag protection could be limited. Common causes include an internal ECU circuit failure, unstable power supply, or poor ground connection. The recommended fix is to visit an authorised dealer for diagnosis and repair. Typical repair cost data is available from recorded cases.
Likely Causes — Atto 3
- 1SRS ECU internal circuit fault: Damaged microprocessor, EEPROM memory, or power regulator chip inside the control unit. Commonly caused by ECU power module breakdown resulting from battery depletion and repeated charging and discharging after long-term vehicle storage.
- 2Abnormal power supply and ground circuits: Poor contact at the ECU constant power (B+) fuse holder, unstable IGN supply voltage, or oxidized ECU ground points or loose bolts causing reference voltage drift, triggering an ECU reset or freeze.
- 3CAN network communication fault: Open or short circuit in the communication line between the SRS ECU and the vehicle CAN bus (powertrain network or body network), or abnormal terminating resistance, preventing the ECU from communicating properly with the VDS diagnostic tool or other modules.
- 4Wiring harness connector issues: Backed-out terminals, spread terminals, or water ingress and oxidation in the SRS harness connector under the steering wheel or inside the centre console (especially in damp environments) interrupt signal transmission or cause crosstalk.
- 5External interference or physical damage: Vehicle wading causes an internal ECU short circuit, or strong electromagnetic interference (such as retrofitted high-power radio equipment) crashes the ECU program or corrupts data.
What To Do
- 1Safety Preparation and Initial Diagnosis: Disconnect the battery negative terminal and wait 3 minutes to discharge residual voltage. Wear an anti-static wrist strap. Connect the BYD VDS diagnostic tool, read the complete fault code list and freeze frame data, and confirm if B16C4 is a current fault (Present) or a history fault (History). Check for accompanying crash sensor or seat belt fault codes.
- 2Power and ground check: Reconnect the battery and measure the voltage at the power supply terminals of the SRS ECU connector (usually under the center console or behind the armrest). Constant power must be 12V (11-14V range), and IGN power must be 12V with the ignition switch in the ON position. Measure the resistance between the ground terminal and the vehicle body; it must be less than 1Ω. Check fuse F4/9 (if applicable) and the fuse holder for enlarged terminals or burn damage.
- 3Communication line check: Use an oscilloscope or multimeter to measure the SRS ECU CAN-H and CAN-L line waveforms. Normal waveforms display a 2.5V reference and 3.5V/1.5V symmetrical square waves. Disconnect the ECU connector and measure the terminal resistance. The resistance must be approximately 60Ω. Check the continuity of the diagnostic communication line (K-line or CAN line) between the OBD diagnostic port and the SRS ECU.
- 4Wiring harness and connector inspection: Focus inspection on the Clock Spring connector under the steering wheel and the mating connectors between the floor harness and instrument panel harness (such as the G09 series). Check for backed-out pins, water ingress, and copper oxidation. Inspect the SRS ECU housing for water marks, cracks, or burn marks.
- 5Software reflash and calibration: If wiring measurements are normal, use VDS to perform an SRS ECU software reflash or rewrite the configuration code. After battery replacement or power disconnection, some models require seat occupancy sensor calibration or crash sensor zero-point calibration.
- 6Component Replacement and Verification: If the above steps fail, replace the SRS ECU assembly. Code the new ECU to write the vehicle VIN and configuration parameters. After installation, clear the fault codes and perform a system self-check (typically cycle the ignition switch 3 times). Finally, perform a simulated crash test (substitute the airbag with a dedicated resistor; never measure the airbag connector directly) to verify system function.
Real-World Cases
BYD Qin/Tang series torque sensor main signal fault
**Symptoms:** While driving, the steering wheel suddenly got heavy and the EPS warning light came on the dashboard. Power steering assistance cut in and out, particularly at low speeds or when turning the wheel while stationary.
**Diagnosis:** Connected the VDS diagnostic tool and retrieved fault codes C1611 and C1612, related to the torque sensor. Battery voltage read normal. Measured 12.3V at pin 1 of the electric power steering control unit connector ER04. Disconnected the EPS control unit and torque sensor connectors and checked continuity between them. Found an open circuit between ER04 pin 3 and the torque sensor. Inspected the wiring harness routing and found the retaining clip under the steering wheel had come loose. Prolonged steering movement caused the internal copper wires to break.
**Resolution:** Repaired the broken wiring harness: resoldered the wires and applied insulation and waterproofing. Re-secured the harness retaining clip so the steering does not pull on the wiring. Cleared the fault codes and calibrated the torque sensor using dedicated equipment. Test drove the vehicle and confirmed the EPS function returned to normal.
Original source ↗BYD Song DM CAN network communication fault (including EPS-related)
Symptoms: The vehicle occasionally stalls while driving but restarts normally. When it stalls, the instrument cluster displays multiple warning lights including EPS, engine and transmission.
Diagnosis: Ran a VDS system scan. Found no current DTCs, but history records showed communication faults between the engine control unit (ECU) and transmission control unit (TCU). Deep-scanned the entire CAN network and found the electric power steering (EPS) control unit was intermittently dropping offline. Inspected the engine bay earth points and found the ECU earth bolt loose, with excessive contact resistance. Measured the EPS control unit supply voltage and found it dropped below 8.5V when the engine bay earth connection was poor.
Resolution: Retightened all earth bolts for the ECU and EPS control unit and applied conductive paste. Inspected and cleaned oxidation off the earth points, sanding them to bare metal. Checked power and earth connections for other CAN network modules and verified no loose joints. Road-tested for 20 km and confirmed the fault did not recur.
Original source ↗BYD Surui EPS steering angle sensor not calibrated
Symptoms: After replacing the steering column assembly, the steering wheel sits centred but the vehicle pulls to one side, the EPS warning light flashes, and steering assistance is asymmetric between left and right.
Diagnosis: Scanned for DTCs, which indicated 'steering angle sensor not calibrated' or signal abnormal. Inspected the steering column installation position and found the steering wheel was not fully centred during assembly. Used a diagnostic scan tool to read the steering angle data stream, which showed 15° with the steering wheel centred (normal range is 0±5°). Inspected the wiring harness connector between the torque sensor and EPS ECU and found the plug not fully locked.
Repair: Removed the steering column and reinstalled it with the steering wheel in the dead-centre position. Fully seated the torque sensor wiring harness connector and confirmed the locking clip engaged. Used the dedicated BYD diagnostic tool to carry out 'steering angle sensor calibration' and 'torque sensor zero-point calibration'. Performed a four-wheel alignment to ensure toe values are within specification. Road tested the vehicle and confirmed good steering wheel self-centring and no pulling to either side.
Original source ↗BYD F3 wheel speed sensor fault causing abnormal steering
Symptoms: ABS and EPS warning lights illuminated simultaneously while driving, with intermittent loss of power steering assistance and erratic speedometer readings.
Diagnosis: Retrieved DTCs indicating left front wheel speed sensor signal fault (C0032) and EPS communication error. Measured left front wheel speed sensor resistance at 1750Ω (specification 1780±150Ω); reading within acceptable range. Inspected sensor connector and wiring harness and found no visible damage. Lifted vehicle and examined driveshaft wheel speed sensor tone ring; found two missing or deformed teeth.
Fault analysis: Abnormal wheel speed signal caused ABS/ESP system to misinterpret vehicle status, affecting EPS assistance characteristic curve.
Resolution: Replaced left front driveshaft assembly (includes wheel speed sensor tone ring). Cleared DTCs from ABS and EPS systems. Performed sensor signal waveform test to confirm normal wheel speed signal. Road test confirmed ABS and EPS functionality returned to normal.
Original source ↗BYD Song MAX stalls while driving with EPS fault
Symptoms: Intermittent stalling while driving; restarts normally. EPS warning light illuminates when the fault occurs, and the steering wheel suddenly becomes heavy.
Diagnosis: Customer recorded a video of the fault, which showed multiple warning lights on the instrument cluster before stalling. Scanned the vehicle with VDS and found a communication fault between engine ECU and TCU, plus a low voltage history code in the EPS control unit. Inspected the engine compartment fuse box and found the F4/9 fuse (EPS power supply) socket had enlarged, causing poor contact. Measured EPS supply voltage during the fault; it dropped sharply from 12V to below 6V, causing the control unit to restart. Traced the wiring harness and found pins 14 and 15 of connector G09 had backed out, causing a loose connection in the power supply circuit.
Solution: Replaced the F4/9 fuse and fuse holder, ensuring tight contact. Repaired the backed-out pins at the G09 wiring harness connector and re-crimped the terminals. Inspected the VCU and rear compartment power distribution box circuits. Extended road testing confirmed the fault was completely resolved.
Original source ↗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.