AU
B16A4-00

DTC B16A4-00 indicates the airbag control unit (SRS ECU) detects an internal or systemic fault — Seal U

Safety System

DTC B16A4-00 indicates the airbag control unit (SRS ECU) detects an internal or systemic fault.

As the core control module of the safety system, the SRS ECU monitors crash sensor status, controls the firing and deployment of airbags and seat belt pretensioners, and communicates with the vehicle CAN network.

This fault indicates potential hardware damage, software malfunction, or power/communication interruption in the ECU’s internal microprocessor, memory, power regulation circuit, or communication interface.

When this DTC triggers, the SRS system enters fail-safe mode.

All airbags (driver, passenger, side, and curtain) and seat belt pretensioners may fail to deploy during a collision.

The instrument cluster continuously illuminates the airbag warning light, severely compromising passive safety protection.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1SRS ECU internal hardware fault: Short or open circuit in a capacitor, resistor, processor chip, or ignition drive circuit on the control unit internal circuit board due to aging, overheating, or manufacturing defects.
  • 2Abnormal power supply: unstable constant power (B+) or ignition power (IG) voltage (below 9V or above 16V), excessive contact resistance in the power supply circuit, blown fuse, or loose or corroded ground points G501/G502
  • 3CAN bus communication fault: A short circuit between the H and L lines of the powertrain CAN or body CAN, a short to ground or power, an open circuit, or abnormal terminating resistance prevents the ECU from communicating normally with the vehicle network.
  • 4Software/data fault: ECU internal EEPROM data corruption, incomplete program flashing, software version incompatible with hardware, or vehicle modifications (such as installing aftermarket electronic equipment) causing signal interference.
  • 5Physical damage or water ingress: Vehicle wading, liquid intrusion during interior cleaning, or collision damage causing ECU housing seal failure, resulting in internal circuit corrosion or cracked solder joints.
What To Do
  • 1
    Fault Confirmation and Freeze Frame Recording: Use a BYD VDS2000 or Launch X-431 diagnostic tool to read the complete DTC. Record the freeze frame data (vehicle speed, voltage, and temperature at the time of the fault). Check for related fault codes (such as U0140-00 BCM communication fault and B16A5-00 internal fault).
  • 2
    Power and ground circuit inspection: Disconnect the battery negative terminal, wait 3 minutes, then unplug the SRS ECU connector. Measure the voltage between connector pin 1 (B+ constant power) and ground, and between pin 17 (IG power) and ground (should be 12V ± 0.5V). Measure the resistance between the ground pin (e.g., pin 34) and body ground (should be less than 1Ω). Check fuses SB03/SB08.
  • 3
    CAN communication circuit check: Turn the ignition ON. Measure the voltage at diagnostic connector pins 6 (CAN-H) and 14 (CAN-L) (normally around 2.5V, with H slightly higher and L slightly lower). Use an oscilloscope to verify the waveform is a standard CAN signal (no noise, no flatline). Measure the resistance between the corresponding pins at the ECU connector (around 60Ω, terminating resistance).
  • 4
    ECU appearance and connection check: Check the ECU mounting location (usually under the center console or inside the center armrest box) for water stains, corrosion, or impact damage. Verify the connector locking tab is intact and the terminals are not backed out or oxidized.
  • 5
    Software reflash attempt: If circuit checks are normal, perform an ECU software reflash (SPS programming). Rewrite the ECU program using the latest software version. This resolves some faults caused by software bugs.
  • 6
    ECU replacement and matching: If the previous steps fail, replace the SRS ECU with the same part number (e.g., 5A-3636100 series). Before installation, verify the vehicle VIN matches the old ECU data. After replacement, perform Online Matching, Coding, and crash sensor Calibration.
  • 7
    System verification: Clear the fault code and perform a system self-check (turn the ignition ON and verify the airbag warning light turns off after 6-8 seconds). Perform a simulated crash test (use the diagnostic tool to execute the crash output test and verify each circuit resistance is within 2.0-3.0 Ω). Road test the vehicle to confirm the fault does not recur.
Real-World Cases
BYD DTC AI Analysis

Qin EV300: airbag warning light on, DTC B16A4-00 (Power Supply Voltage Abnormal) stored.

✓
Vehicle: 2017 Qin EV300, 80,000 km. Symptoms: Airbag warning light remained on after startup. Scan tool retrieved DTC B16A4-00. Diagnosis: Measured B+ voltage at SRS ECU connector at only 8.2V — low. Traced the cause to severe oxidation on the battery positive terminal. Also found the SB03 fuse (10A) base in the instrument panel distribution box burnt, causing poor contact. Repair: Cleaned and tightened battery terminals. Replaced the distribution box base and fuse. B+ voltage restored to 12.4V. Cleared fault codes and system returned to normal.
BYD DTC AI Analysis

Qin 100 SRS system failure after wading, DTC B16A4-00 (ECU water ingress corrosion).

✓
Vehicle: 2017 BYD Qin 100. Caught in heavy flooding with water above the door sills. Symptoms: After the vehicle dried out, the dashboard displayed "Check Airbag System" on startup. Scan tool retrieved DTC B16A4-00 and multiple crash sensor communication faults. Investigation: Removed the SRS ECU from beneath the centre console. Found the housing seal had perished, with clear water staining and pin corrosion on the internal circuit board. Repair: Fitted a new SRS ECU, cleaned and dried the wiring harness connectors, performed online coding and sensor calibration. Fault resolved.
BYD DTC AI Analysis

BYD Qin EV450 logged DTC B16A4-00 (CAN bus interference) following accident repairs

✓
Vehicle: 2018 BYD Qin EV450, after front-end collision repair. Symptom: After replacing the front bumper and wiring harness during the accident repair, the SRS warning light came on with code B16A4-00. Diagnosis: ECU power supply measured normal, but the oscilloscope detected significant noise on the CAN waveform. Inspection found the front compartment harness had chafed against the AC high-pressure line due to its routing during the repair, wearing through the CAN-H wire insulation and making intermittent contact with the metal pipe. Repair: Repaired the damaged CAN harness, rerouted and secured the wiring to avoid further interference. Cleared the fault codes; the system returned to normal.
BYD DTC AI Analysis

Qin 80 software version defect causing intermittent B16A4-00

✓
Vehicle: 2017 BYD Qin 80, 50,000 km. Symptoms: Intermittent airbag warning light illumination that cleared after cycling the ignition. Scan tool retrieved stored DTC B16A4-00. Troubleshooting: Checked power supply, ground and CAN bus wiring – all normal. ECU showed no physical damage. Consulted BYD technical service bulletin (TSB) and confirmed this production batch has an SRS ECU software bug that triggers false internal fault codes during specific voltage fluctuations. Repair: Performed ECU software upgrade using VDS (from V1.02 to V1.05). Monitored for one month after the upgrade with no recurrence.
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.