AU
B16A4

DTC B16A4 indicates the airbag control unit (SRS ECU) detected an internal system fault or critical support circuit abnormality — Seal U

Safety System

DTC B16A4 indicates the airbag control unit (SRS ECU) detected an internal system fault or critical support circuit abnormality.

As the core of the passive safety system, the ECU processes real-time data from the front and rear crash sensors, side pressure sensors, and accelerometers.

It determines airbag deployment, seat belt pretensioner activation, and high-voltage interlock disconnection.

This DTC triggers when the ECU self-diagnostic detects an internal CPU processing error, an EEPROM memory checksum failure, a transient voltage drop at the power supply monitoring circuit, or a communication interruption between the master and slave safety chips.

In this condition, the system enters fail-safe mode.

This mode may disable all airbag deployment functions and fails to guarantee normal protection logic during a collision, creating a severe safety hazard.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Poor solder joint on the SRS ECU internal PCB or main control chip hardware fault (such as Infineon or Renesas series)
  • 2ECU power supply circuit fault: Excessive resistance in the constant power (+B) circuit, poor contact at the ignition switch power supply (IG1), or corrosion at ground terminals G301/G302 causing voltage fluctuations.
  • 3CAN bus communication fault: Electromagnetic interference disrupts the Power CAN or Chassis CAN signals between the SRS ECU, instrument cluster, and VCU, or a terminal resistor mismatch causes data frame loss.
  • 4Crash sensor internal short circuit: A short to ground or short to power in the front or side crash sensor signal harness triggers the ECU analog input port overload protection.
  • 5Internal ECU accelerometer damaged by a previous collision, or ECU housing seal failure caused by vehicle wading, resulting in oxidation and corrosion of the internal circuit board.
What To Do
  • 1
    Use the BYD dedicated diagnostic tool (ED400 or VDS) to read the complete DTC list, check for accompanying B16A3 (SRS ECU internal fault), U0155 (lost communication with instrument cluster), or B1B00-series sensor fault codes, and record the freeze frame data.
  • 2
    Check battery voltage and body ground points. Measure voltage at SRS ECU connector terminal 1 (+B constant power) and terminal 9 (IG1 power). Verify static voltage is ≥12.4 V and dynamic voltage is ≥9 V. Verify ground circuit resistance is <1 Ω.
  • 3
    Disconnect the battery negative terminal and wait 3 minutes for the capacitors to discharge. Inspect the ECU connector (located under the center console or behind the center armrest box) for pin oxidation, backed-out pins, or wiring harness wear. Clean with electrical contact cleaner if necessary.
  • 4
    Use an oscilloscope to measure CAN-H (2.5-3.5V) and CAN-L (1.5-2.5V) waveforms, check for abnormal pulses or shorts to power/ground, and verify network topology integrity.
  • 5
    Check the SRS ECU software version against the BYD TIS technical bulletin. If a newer software version is available (such as a patch for B16A4), update the software and reconfigure the vehicle VIN and airbag parameters.
  • 6
    If the fault code persists, replace the SRS ECU assembly (select the part number according to the vehicle model, e.g., EG-5820100 for Yuan EV). After replacement, perform: ① Coding ② Crash sensor calibration ③ System self-diagnostic test.
  • 7
    Use the diagnostic tool to perform the 'Airbag System Check' function, simulate a crash signal (without actual ignition), and confirm the system status returns to normal. Clear the fault code and road test to verify.
Real-World Cases
BYD DTC AI Analysis

BYD Tang DM-i: Power fluctuation causes intermittent SRS ECU fault

✓
A 2021 Tang DM-i with 30,000 km on the clock showed an intermittent "Check SRS System" warning on the dash, with fault codes B16A4 and U0155 present. Diagnosis revealed an aftermarket dash cam drawing power from the SRS fuse circuit, causing voltage to drop to 8.2V on startup. Restored the factory power circuit, fitted a filter capacitor to the SRS ECU power supply, and upgraded the ECU software to V2.3 to improve voltage tolerance. Fault resolved.
BYD DTC AI Analysis

Flooded Yuan EV: SRS ECU Internal Corrosion Repair Case

✓
2019 Yuan EV logged DTC B16A4 after a water wading incident. Disassembly found the SRS ECU mounted under the floor. The internal PCB showed copper corrosion, and the accelerometer chip solder joints had cracked. Because the ECU is a safety-critical component and internally potted, it cannot be repaired; we fitted a new ECU. Recalibrated the zero points for the three collision sensors (left front, right front, centre) using the diagnostic scanner, and completed the high-voltage interlock loop check. Total labour time: 4.5 hours.
BYD DTC AI Analysis

CAN bus interference caused B16A4 fault in BYD Song MAX

✓
After the accident repair, the SRS warning light stayed on and stored DTC B16A4 along with multiple U communication faults. Inspection found the wiring harness was not secured properly during the repair, causing the SRS ECU CAN bus wiring to run parallel to the orange high-voltage wiring for an excessive distance. This caused electromagnetic coupling interference. Fix: re-routed the wiring to maintain ≥200 mm separation between the CAN lines and high-voltage cables, added a ferrite core at the ECU end, cleared the fault codes, and monitored the vehicle for 200 km with no recurrence.
BYD DTC AI Analysis

Failure to configure the instrument cluster after replacement caused an SRS fault on the Qin Pro EV

✓
Following instrument cluster replacement at a dealership, a 2020 Qin Pro EV set an SRS B16A4 fault with the airbag warning light remaining constantly on. Diagnosis revealed the replacement cluster lacked the correct security system configuration code, causing a failed security authentication handshake with the SRS ECU. Repair procedure: Using the VDS diagnostic tool, navigate to 'Special Functions - SRS Configuration', enter the correct configuration code (based on vehicle trim: 5-airbag or 6-airbag variant), and perform online matching to clear the fault.
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.