AU
B17FF

DTC B17FF indicates an abnormal vehicle speed pulse signal at the airbag control unit (SRS ECU) — Seal U

Safety System

DTC B17FF indicates an abnormal vehicle speed pulse signal at the airbag control unit (SRS ECU).

In BYD electronic architecture, wheel speed sensors typically generate the vehicle speed signal.

The ABS/ESP control unit processes this signal and transmits it via the CAN bus to the instrument cluster and SRS module.

The SRS system relies on accurate vehicle speed data for crash algorithm decisions: low-speed collisions (<25km/h) normally do not trigger the front airbags, while medium- and high-speed collisions require precise vehicle speed data to calculate airbag deployment timing and ignition level.

When the SRS module detects the vehicle speed pulse signal is missing, out of range, erratic, or inconsistent with CAN bus vehicle speed data, it logs DTC B17FF and illuminates the airbag warning light.

The system enters a degraded mode, potentially causing unintended or delayed airbag deployment during a collision.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Wheel speed sensor fault or dirty/damaged signal tone ring causing abnormal pulse signal.
  • 2CAN bus communication fault between the ABS/ESP control unit and the SRS module (open circuit, short circuit, or signal interference)
  • 3SRS control unit internal vehicle speed signal processing circuit fault (damaged ADC converter or signal conditioning circuit)
  • 4Instrument panel power distribution module or gateway forwarding abnormal vehicle speed signal.
  • 5Vehicle modifications (such as installing an aftermarket dashcam or GPS tracker) interfering with the vehicle speed signal line.
What To Do
  • 1
    Use the BYD VDS diagnostic tool to read all fault codes, check for accompanying U-class communication faults or C-class chassis faults, and view the 'Vehicle Speed Validity' status in the live data stream.
  • 2
    Check the four wheel speed sensors and signal tone rings: clean metal filings from the sensor heads, inspect the tone rings for missing teeth or deformation, and measure the sensor resistance (normal: 1.3-1.8 kΩ) and output AC voltage (0.1-3 V while rotating the wheel).
  • 3
    Measure resistance (60Ω±5Ω) and voltage (CAN-H 2.6-2.8V, CAN-L 2.2-2.4V) at diagnostic connector terminals 6# (CAN-H) and 14# (CAN-L), and check for a short to ground or power.
  • 4
    Disconnect the battery negative terminal. Check the 12V constant power, ignition switch power, and ground points at the SRS module connector (usually under the center console or in front of the gear selector) for oxidation or looseness. Measure the power supply voltage; it must be ≥12V.
  • 5
    Check the SRS module software version. If an update is available, perform the software upgrade first. If the fault persists, keep the vehicle stationary and observe the vehicle speed value in the data stream. If the speed value fluctuates abnormally while the ABS data stream is normal, replace the SRS control unit.
  • 6
    Clear the fault code and perform a road test. While driving at steady speeds of 20 km/h, 40 km/h, and 60 km/h, compare the instrument cluster vehicle speed with the ABS and SRS vehicle speed data on the diagnostic tool. The difference among the three must be ≤2 km/h.
Real-World Cases
BYD DTC AI Analysis

Faulty left front wheel speed sensor caused loss of vehicle speed pulse

✓
A 2019 BYD Yuan EV presented with the SRS warning light permanently illuminated. Retrieved DTC B17FF. Found an internal open circuit in the left front wheel speed sensor; resistance measured infinite (normal approximately 1.5kΩ). This vehicle uses the left front wheel as the primary vehicle speed signal source, and the faulty sensor prevented the SRS module from receiving pulse signals. Replaced the left front wheel speed sensor. The vehicle speed signal in the data stream returned to normal, and the DTC cleared and did not return.
BYD DTC AI Analysis

Loose CAN bus connection causing intermittent vehicle speed fault

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2021 BYD Tang DM. Airbag warning light occasionally illuminates on startup and goes off after driving for a while. Scanner shows B17FF as an intermittent fault. Inspection of the CAN wiring between the Gateway Controller (GWC) and SRS module found a loose connection at the white connector (BJ03) below the A-pillar. The male terminal had backed out, causing an intermittent CAN-H signal. Repaired the connector and applied conductive grease. After 20 km of continuous bumpy road testing, the fault did not return.
BYD DTC AI Analysis

Aftermarket GPS tracker interfering with vehicle speed signal

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2020 BYD Qin petrol variant. DTC B17FF set after installing an aftermarket GPS tracker. The shop had paralleled the tracker’s speed signal wire with the factory vehicle speed line, shunting the pulse signal and dropping the amplitude received by the SRS module. Disconnected the aftermarket device and restored the factory harness insulation; fault cleared. If you need vehicle speed data, get it from the CAN bus via a protocol box—do not parallel the pulse signal wire.
BYD DTC AI Analysis

Internal ADC circuit fault in the SRS control unit

✓
2019 BYD Song MAX. DTC B17FF occurred multiple times. Replaced wheel speed sensor and checked wiring harness without success. Further inspection revealed the SRS module showed 255km/h (maximum value) in the data stream while stationary—clear hardware fault. Disassembled the SRS control unit and found a dry joint on the ADC conversion chip (MPC560x series) at the vehicle speed signal input pin. Resoldered the chip pins, applied conformal coating, and resolved the fault. Avoided replacing the expensive SRS assembly.
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.