AU
B16C4

DTC B16C4 indicates an internal fault or severe communication failure in the airbag control unit (SRS ECU) — Seal U

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.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1SRS ECU internal circuit fault: Damaged internal microprocessor, EEPROM memory, or power regulator chip inside the control unit. Repeated charging and discharging of a depleted battery after extended vehicle storage commonly causes ECU power module breakdown.
  • 2Power supply and ground circuit fault: Poor contact at the ECU constant power (B+) fuse holder, unstable IGN supply voltage, or an oxidized ECU ground point or loose bolt causing reference voltage drift, triggering an ECU reset or freeze.
  • 3CAN network communication fault: An open or short circuit in the communication line between the SRS ECU and the vehicle CAN bus (powertrain or body network), or abnormal terminal resistance, prevents the ECU from communicating properly with the VDS diagnostic tool or other modules.
  • 4Wiring harness connector issues: Backed-out pins, spread terminals, or water ingress and oxidation at the SRS wiring harness connector below the steering wheel or inside the centre console (especially in damp conditions), causing signal interruption or 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) causes an ECU program crash and data corruption.
What To Do
  • 1
    Safety 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. Confirm if B16C4 is a current fault (Present) or a history fault (History). Check for accompanying crash sensor or seat belt fault codes.
  • 2
    Power and ground check: Reconnect the battery. Measure the voltage at the power supply terminals of the SRS ECU connector (usually located under the center console or behind the armrest box). Constant power must be 12V (11-14V range). IGN power must be 12V with the ignition switch in the ON position. Measure the resistance between the ground terminal and the vehicle body; resistance must be less than 1Ω. Check fuse F4/9 (if applicable) and the fuse holder for enlarged terminals or burn damage.
  • 3
    Communication line inspection: Use an oscilloscope or multimeter to measure the SRS ECU CAN-H and CAN-L waveforms. A normal waveform displays a 2.5V reference and a 3.5V/1.5V symmetrical square wave. Disconnect the ECU connector and measure the terminal resistance. The resistance should 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.
  • 4
    Wiring harness and connector inspection: Focus 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 connectors). Check for backed-out terminals, water ingress, and green oxidation. Inspect the SRS ECU housing for water marks, cracks, or burn marks.
  • 5
    Software reflash and calibration: If wiring measurements are normal, use VDS to reflash the SRS ECU software or rewrite the configuration code. Some models require seat occupancy sensor calibration or crash sensor zero-point calibration after replacing the battery or disconnecting the power supply.
  • 6
    Component 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 (usually by cycling the ignition switch 3 times). Finally, perform a simulated crash test (use a dedicated resistor in place of the airbag; do not directly measure the airbag connector) to verify system function.
Real-World Cases
BYD DTC AI Analysis

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 DTC AI Analysis

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 DTC AI Analysis

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 DTC AI Analysis

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 DTC AI Analysis

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 ↗
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.