AU
B16C5

DTC B16C5 indicates an internal fault or critical function failure in the airbag control unit (SRS ECU) — Seal U

Safety System

DTC B16C5 indicates an internal fault or critical function failure in the airbag control unit (SRS ECU).

The SRS ECU is the core controller of the safety system.

It monitors vehicle collision acceleration, processes crash signals, and controls the ignition triggering of protective devices such as airbags and seat belt pretensioners.

The following conditions trigger this DTC: ECU internal main control chip (MCU) self-test failure, abnormal internal acceleration sensor signal, ignition circuit driver fault, EEPROM read/write error, or persistent ECU power supply voltage or communication bus abnormalities.

When this fault occurs, the SRS system may enter fail-safe mode, preventing normal airbag deployment during a collision or, in extreme cases, causing unintended deployment.

This critical fault directly compromises occupant safety.

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Cases Logged
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Causes
Likely Causes — Seal U
  • 1SRS ECU internal hardware damage: Aging, overheating, or electromagnetic interference caused functional failure of the main control chip, internal crash sensor, or ignition driver circuit. This is common in 2018-2020 batches of Yuan/Song models.
  • 2Power supply system fault: excessive contact resistance in the constant power (B+) circuit, unstable IGN supply voltage (below 9V or above 16V), or poor ground circuit causing abnormal ECU reset, mostly occurring with a low battery or after installing an aftermarket audio system.
  • 3CAN bus communication fault: Lost communication or message timeout between the SRS ECU and the vehicle network. Possible causes include a wiring harness short circuit, connector water ingress (e.g., poor sealing at the wiring harness hole below the A-pillar), or an abnormal terminating resistor.
  • 4Collision data locked: The vehicle experienced a collision, the ECU internally recorded the collision event without a proper reset, or a technician installed a used ECU without data synchronization.
  • 5Software/calibration error: Corrupted ECU internal Flash data, calibration parameter mismatch with vehicle configuration, or interrupted flashing process causing program integrity check failure.
What To Do
  • 1
    Use VDS2000 or the latest BYD diagnostic tool to read all DTCs, check for accompanying fault codes (such as U-series communication faults or B16XX-series sensor faults), and record freeze frame data (vehicle speed, voltage, and temperature at the time of the fault).
  • 2
    Perform a key cycle test: disconnect the battery negative terminal for 5 minutes, reconnect it, and check if the fault code changes to a history code. If it remains a current code, check the SRS ECU power supply: measure connector pin 1 (B+) for 12V constant power, pin 2 (IGN) for 12V with the ignition switch ON, and pin 3 (GND) for less than 1Ω resistance to ground.
  • 3
    Check CAN communication: Measure diagnostic port pins 6 (CAN-H) and 14 (CAN-L). Sleep mode voltage is approximately 0V. After wake-up, CAN-H measures 2.5-3.5V and CAN-L measures 2.5-1.5V. The waveform must be a symmetrical square wave. If voltage is abnormal, inspect the SRS wiring harness section by section (located under the center console, where A/C condensate leaks easily corrode it).
  • 4
    Visually inspect the ECU exterior: Remove the center console lower trim panel and check the SRS ECU housing for cracks, signs of water ingress (white corrosion), or impact deformation. If signs of water exposure are present, replace the ECU and repair the water leak point (commonly a detached sunroof drain hose).
  • 5
    Software repair attempt: If wiring is normal, use the diagnostic tool to perform 'SRS System Calibration' or 'Configuration Code Writing' (enter the vehicle VIN and configuration code). Updating the SRM (Supplemental Restraint System Module) software version resolves some B16C5-00 faults (verify the part number supports software upgrades).
  • 6
    Replace SRS ECU: If the previous steps fail, order a new ECU (verify high-spec/low-spec and with/without side airbag configurations). After installation, perform 'crash sensor calibration' and 'system self-learning', and clear all history fault codes. Recycle the old ECU as hazardous waste.
Real-World Cases
BYD DTC AI Analysis

B16C5-00 cannot be cleared after collision on Tang DM

✓
2021 Tang DM. After a minor front-end collision and front bumper replacement, the instrument cluster kept displaying the airbag warning light. Scan tool retrieved active DTC B16C5-00, accompanied by B1650-00 (driver airbag module). Inspection found no external damage to the SRS ECU, but it had internally logged the collision and locked. Online reset attempt failed. Replaced the SRS ECU (part number SC-3636900-B1), programmed the vehicle configuration, then completed ‘Restraint System Configuration’ and ‘Longitudinal Acceleration Sensor Calibration’. Fault resolved.
BYD DTC AI Analysis

Wading through water caused ECU internal short circuit in BYD Yuan EV

✓
2019 Yuan EV535. After heavy rain, drove through standing water. The next day the airbag warning light came on. DTC B16C5-00 stored. Removed and inspected the SRS ECU under the floor — the housing seal had aged, letting water in, and the circuit board had green copper corrosion. Measured a B+ short to ground. Replaced the ECU, cleaned up water on the wiring harness, and unclogged the sunroof drains (blockage let water into the A-pillar). Fault fully resolved.
BYD DTC AI Analysis

BYD Song Max intermittent B16C5 with CAN communication fault

✓
2019 Song MAX. Airbag warning light intermittently illuminated when driving on rough roads. Scan tool showed intermittent B16C5 and U0155-00 (lost communication with SRS). Inspection found pin 10 (CAN-H) of the 32-pin SRS ECU connector beneath the centre console had backed out, causing poor contact. Repaired the pin and applied conductive grease. Secured the wiring harness to prevent strain. Fault has not recurred.
BYD DTC AI Analysis

Aftermarket modifications caused abnormal power supply voltage in Qin Pro

✓
2020 Qin Pro petrol. Owner fitted a dashcam by tapping power from the airbag fuse, causing SRS ECU supply voltage fluctuations (minimum measured 8.2 V). Diagnosed DTC B16C5-00 and multiple voltage-related history codes. Restored the factory fuse circuit and ran a dedicated power feed for the aftermarket device. Cleared fault codes; system operation returned to normal. Warning: Never tap power from SRS system fuses (typically SB03, SB07).
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.