AU
B16C3

DTC B16C3 indicates a functional fault in the airbag system electronic control unit (SRS_ECU) — Seal U

Safety System

DTC B16C3 indicates a functional fault in the airbag system electronic control unit (SRS_ECU).

The SRS_ECU is the core controller of the safety system.

It monitors crash sensor signals in real time, processes crash algorithms, controls the deployment timing of the airbags and seat belt pretensioners, and manages the communication network of the entire passive safety system.

This DTC usually indicates an internal processor or memory fault in the ECU, or a loss of communication between the ECU and the vehicle CAN network.

An intermittent open circuit in the ECU power supply or ground circuit can also trigger this fault.

This fault causes the airbag system to enter fail-safe mode.

The airbags may fail to deploy in a collision, creating a serious safety risk.

3
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Abnormal SRS ECU power supply circuit: Includes a blown constant power (B+) circuit fuse, poor relay contact, or unstable IGN supply voltage, causing ECU operating voltage to drop below 9V or exceed 16V.
  • 2CAN bus communication fault: A short circuit, open circuit, or abnormal terminal resistance in the communication line between the SRS ECU and the vehicle network (powertrain CAN or dedicated safety CAN) prevents the ECU from communicating normally with the instrument cluster, VCU, and other modules.
  • 3ECU internal hardware damage: Control unit internal memory data corruption, processor clock fault, or collision detection circuit self-check failure, usually resulting from electromagnetic interference, overvoltage, or component aging.
  • 4Sensor circuit short: Short to ground or power in the front impact sensor, side impact sensor, or clock spring wiring triggers the ECU protective shutdown function.
  • 5Software calibration error: Corrupted ECU internal calibration data, incomplete flashing, or failure to perform online configuration (coding) after parts replacement causes abnormal system recognition.
What To Do
  • 1
    Safety preparation and initial diagnosis: Disconnect the high-voltage system (for new energy vehicles) and wait 5 minutes to allow capacitors to discharge. Use the BYD VDS diagnostic tool to read all fault codes. Confirm if B16C3 is a current fault (Active) and check for accompanying communication fault codes (such as the U01XX series). Record freeze frame data and review vehicle speed, voltage, and other parameters from when the fault occurred.
  • 2
    Power and ground check: Check for blown SRS system fuses (e.g., F1/15, F2/13) in the front compartment fuse box. Disconnect the battery negative terminal, wait 90 seconds, and unplug the SRS ECU connector. Measure the voltage between pin 1 (constant power +B) and ground (must equal battery voltage). Measure the resistance between the ground pin and body ground (must be less than 1Ω). Inspect the connector for oxidation or backed-out pins.
  • 3
    Communication line check: Reconnect the battery. Turn the ignition switch to ON (do not start). Use an oscilloscope to measure the waveform at SRS ECU connector CAN-H (pin 6) and CAN-L (pin 14). The standard waveform is a square-wave signal with CAN-H at 2.5-3.5V and CAN-L at 1.5-2.5V. If the voltage is abnormal (e.g., both are close to 0V or 12V), check the wiring harness for a short or open circuit.
  • 4
    Sensor circuit inspection: Check the wiring harness connectors for the front impact sensor (located on the longitudinal beam) and the side airbag sensor. Measure the sensor resistance to confirm it is within the standard range (typically 2-3 kΩ). Check the steering wheel clock spring (spiral cable) continuity. Verify the driver-side airbag circuit resistance is normal (2-3 Ω) to rule out ECU misjudgment caused by a sensor circuit fault.
  • 5
    ECU replacement and calibration: If all circuit measurements are normal, update the SRS ECU software (if BYD has released a relevant upgrade package). If the fault persists, replace the SRS ECU. After replacement, use VDS to perform 'Online Coding' and 'Calibration', and enter the correct vehicle VIN and safety configuration code. Finally, clear the fault codes, perform a system self-check (the indicator light should turn off), and perform a simulated crash test (using dedicated equipment) to verify system functionality.
Real-World Cases
BYD DTC AI Analysis

BYD Song MAX: Faulty evaporator temperature sensor caused air conditioning failure

✓
Poor A/C cooling, vent temperature not low enough, intermittent compressor operation. VDS scan found DTC B2A2F09 (evaporator temperature sensor fault). Data stream showed evaporator temperature fixed at –40°C (clearly abnormal). Further inspection revealed backed-out pins in the evaporator temperature sensor connector causing poor contact. Re-secured the sensor connector pins and cleared the fault code. Data stream returned to normal and A/C cooling function was restored.
Original source ↗
BYD DTC AI Analysis

BYD Qin DM: A/C compressor not starting

✓
After turning on the A/C, no cold air blew from the vents, the compressor did not start, and the recirculation/fresh air switching was inoperative. The owner initially ran an OBD scan and found multiple communication fault codes in the climate control system. Inspection at the dealership confirmed an abnormal power supply to the HVAC control module. The A/C compressor fuse in the engine bay fuse box had blown. Replacing the fuse did not fix the fault. Further investigation revealed an internal short circuit in the A/C pressure switch, triggering system protection. After replacing the pressure switch, the compressor worked normally and the fault was fully resolved.
Original source ↗
BYD DTC AI Analysis

BYD Song Pro automatic A/C temperature control fault

✓
The fault symptoms were a significant discrepancy between the set temperature and actual air outlet temperature, a noticeable temperature difference between the left and right air vents, and abnormal fan speed adjustment in AUTO mode. A dealership technician used a scan tool to retrieve fault codes related to the temperature blend door actuator. Checking the live data from each temperature sensor, the technician found the cabin temperature sensor reading deviated from the actual ambient temperature by about 15°C. The technician diagnosed a failed cabin temperature sensor (located inside the dashboard). The repair involved replacing the cabin temperature sensor and recalibrating the blend door actuator using the scan tool. After the repair, temperature control accuracy returned to normal.
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.