AU
B174400

The BYD Qin PRO series defines DTC B174400 as "Crash Output Fault", a critical safety fault in the SRS airbag system — Seal U

Safety System

The BYD Qin PRO series defines DTC B174400 as "Crash Output Fault", a critical safety fault in the SRS airbag system.

This indicates the airbag control unit (ACU) detects a circuit abnormality when attempting to output a crash trigger signal to other vehicle systems.

In BYD new energy vehicles, this specifically refers to a crash signal interaction fault between the ACU, the high-voltage system manager (BMS/VCU), and the body control module (BCM), including: 1) A short to power/ground or an open circuit in the crash output signal line; 2) Damage to the ACU internal crash output driver MOSFET circuit; 3) Abnormal feedback signals from receiving actuators (e.g., high-voltage power cut-off relay, fuel pump cut-off relay, and door unlock relay).

During a collision, this fault prevents high-voltage system disconnection (electric shock risk), fuel pump shutoff (fire risk), and automatic door unlocking (hindering escape and rescue).

This significantly increases the risk of secondary accidents.

Hyundai/Kia vehicles typically define this DTC as "Front Passenger Side Pressure-Type Side Impact Sensor ID Error", indicating cross-brand definition differences.

For BYD systems, prioritize diagnosing the crash output signal circuit and ACU driver circuit faults.

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Cases Logged
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Causes
Likely Causes — Seal U
  • 1Damaged internal crash output driver circuit in the SRS control unit (ACU), or outdated software causing a false warning.
  • 2Crash output signal wiring harness (connected to the high-voltage system manager, BCM, fuel pump relay, etc.) worn at the door hinge, shorted to power/ground, or open circuit.
  • 3Internal short circuit at the collision signal receiving port of the high-voltage system manager or BCM, causing abnormal feedback voltage.
  • 4A previous collision damaged the ACU or related wiring harness and repairs were incomplete, or non-genuine parts caused a signal mismatch.
  • 5Right front door pressure-type side impact sensor (P-SIS) fault, poor connector contact, or lost ID registration, causing the ACU to incorrectly determine collision status.
What To Do
  • 1
    Use a BYD VDS2000 or the latest diagnostic tool to read all SRS system fault codes and freeze frame data. Confirm whether B174400 is a current or historical fault code. Record key information such as vehicle speed and time of occurrence.
  • 2
    Visually inspect the front and sides of the vehicle for signs of collision. Check the SRS control unit (located under the center console or gear selector) for physical condition, mounting security, and water ingress at the connectors.
  • 3
    Measure the continuity of the crash output signal circuits (including the high-voltage interlock cut-off signal wire, fuel pump cut-off signal wire, and door unlock signal wire). Inspect the wiring harness at the right front door hinge for wear or damaged insulation, and test for a short to power or ground.
  • 4
    Check the operating status and feedback circuit resistance of the relevant actuators (high-voltage power cut-off relay, fuel pump relay, central locking actuator). Verify no short circuit exists at the receiving end (normal feedback resistance is typically 2-3kΩ).
  • 5
    If the wiring harness and actuator are normal, perform an ACU self-diagnostic test and check the software version. Upgrade to the latest version if necessary. If the fault persists, replace the airbag control unit, complete online coding, and write configuration parameters.
  • 6
    After repair, perform a crash output function test (use the diagnostic tool to simulate a crash signal). Verify the high-voltage system powers down, the fuel pump stops, and the doors automatically unlock. Clear the fault code and perform a road test to confirm.
Real-World Cases
BYD DTC AI Analysis

Qin Pro DM right front pressure sensor ID error caused abnormal collision output.

✓
The airbag warning light on the instrument cluster stayed on. The scan tool read DTC B174400. The right front door had recently received panel and paint repair. During disassembly, we found the pressure side impact sensor (P-SIS) connector was not fully seated, causing an intermittent connection. The ACU could not correctly identify the sensor ID, triggering the collision output fail-safe mode. We reseated the connector and replaced the pressure sensor. After performing the sensor ID registration with the scan tool, the fault cleared.
Original source ↗
BYD DTC AI Analysis

Qin Pro petrol version door wiring harness wear causes collision signal fault

✓
While driving, the airbag warning light suddenly illuminated, accompanied by intermittent central locking failure. Retrieved fault code B174400. Inspection found the wiring harness at the right front door hinge had suffered fatigue wear from long-term repeated door opening and closing. The pressure sensor communication lines (CAN-H/L) had fractured and were making poor contact, preventing the ACU from receiving correct side impact sensor data. Repaired the damaged wiring harness (re-soldered and wrapped with waterproof tape) and replaced the pressure sensor that may have been damaged by the wiring short circuit. Fault resolved.
BYD DTC AI Analysis

Qin Pro DM: ACU internal driver circuit fault after accident repair

✓
The vehicle sustained front-end collision damage. After the dealership replaced the airbags and sensors, the airbag warning light stayed on and DTC B174400 could not be cleared. Checked the signal wiring harness from the collision output to the high-voltage system manager—it was normal. The high-voltage manager receiving port was normal. Measured no voltage at the ACU output terminal. Determined the collision had damaged the collision output driver chip inside the ACU. Replaced the airbag control unit and completed online matching, coding, and high-voltage system integration. The warning light went out and the simulated collision test functioned normally.
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.