AU
B174A

DTC B174A indicates the measured resistance of the Left Rear Side Airbag ignition circuit is 0 ohms, representing a short circuit fault in the airbag system — Seal U

Safety System

DTC B174A indicates the measured resistance of the Left Rear Side Airbag ignition circuit is 0 ohms, representing a short circuit fault in the airbag system.

Normal airbag igniter resistance ranges from 1.5 to 3.5 ohms.

A 0-ohm reading indicates a short to ground or short to power in the ignition wiring, allowing current to directly bypass the igniter.

This causes the SRS control unit to identify the airbag circuit as faulty, illuminate the airbag warning light, and disable deployment of the Left Rear Side Airbag.

In a side collision, this airbag may fail to inflate normally, severely compromising side impact protection for rear passengers.

This fault represents a loss of airbag ignition circuit integrity.

Repair the system immediately to ensure correct operation of the passive safety system.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Short circuit in left rear side airbag module internal igniter (short circuit in airbag internal bridge wire or moisture ingress)
  • 2Airbag wiring harness short to body ground (seat slide rail wears through the harness insulation, seat frame pinches the harness)
  • 3Water ingress, oxidation, or bent pins in the under-seat or C-pillar airbag connector causing a short circuit between the positive and negative terminals.
  • 4Internal driver circuit fault in the SRS control unit (ACU) or damaged detection chip causing a false alarm.
  • 5Water enters the airbag connector during vehicle wading or interior cleaning, causing a short circuit between terminals.
What To Do
  • 1
    Safety Preparation: Disconnect the battery negative terminal and wait at least 90 seconds to discharge residual power in the SRS system and prevent accidental airbag deployment.
  • 2
    Fault confirmation: Use the BYD VDS or a dedicated diagnostic tool to read the DTC and confirm B174A is a current fault (Present), not a history fault.
  • 3
    Locate component: Remove the left rear seat backrest or C-pillar trim, and locate the left rear side airbag module and yellow SRS connector.
  • 4
    Measure the airbag: Disconnect the airbag connector. Use a dedicated resistance meter to measure the resistance between the two terminals of the airbag module. Normal resistance is 1.5-3.5 ohms. If the resistance is 0 ohms, replace the airbag module.
  • 5
    Wiring harness check: Measure the resistance to ground at the wiring harness side connector terminal. Continuity indicates a short to ground. Inspect the wiring harness for chafing or damage from rubbing against the seat slide rail or metal frame.
  • 6
    Connector inspection: Check the SRS connector under the seat (usually at the seat base or side) for signs of water ingress, oxidation, or deformed pins. Clean or replace the connector if necessary.
  • 7
    ECU check: If the airbag and wiring harness are normal, inspect the SRS control unit connector terminals for corrosion. Measure the circuit drive terminal for a short to ground. Replace the ACU control unit if necessary.
  • 8
    Repair verification: After replacing the faulty component, restore connections and reconnect the battery. Use the diagnostic tool to clear the fault code, perform an SRS self-test, and confirm B174A does not return.
  • 9
    Function test: Use the diagnostic tool to perform an airbag system function test and verify the left rear side airbag circuit resistance is within the standard range (2.0 ± 0.5 ohms).
Real-World Cases
BYD DTC AI Analysis

BYD Song Pro DM-i air conditioning system communication fault

✓
Symptoms: The air conditioning system worked intermittently, and the centre touchscreen displayed “Air Conditioning System Fault”. The fault occurred frequently in hot summer weather and sometimes returned to normal after restarting the vehicle. The owner reported that the fault mostly appeared after the vehicle had been running for 30 minutes. Diagnosis: The technician connected a diagnostic scan tool and retrieved fault codes B174A (Communication Fault between Air Conditioning Control Module and Body Control Module) and U0146 (Lost Communication with Gateway Module). The technician checked the CAN bus voltages and found CAN-H at 2.8V (normal range 2.5–3.5V) and CAN-L at 2.1V (normal range 1.5–2.5V). Although within specification, the voltages fluctuated significantly. Further inspection found oxidation on the air conditioning control module connector and slight corrosion on the pins. Repair: The technician cleaned the air conditioning control module connector pins, applied conductive grease to prevent corrosion, checked and tightened the earth wire (G201), and updated the air conditioning control module software to the latest version (V2.3.5). Follow-up: The fault did not recur during a one-month follow-up.
Original source ↗
BYD DTC AI Analysis

BYD Qin Plus EV heat pump air conditioning fault

✓
**Symptom Description:** The owner of a 2022 Qin PLUS EV reported that the heater blew cold air when switched on, while cooling mode worked normally. No warning lights illuminated on the instrument panel, but the AC panel temperature adjustment failed and only ambient air came out. **Diagnostic Process:** Connected the BYD dedicated diagnostic tool VDS2000 and retrieved DTC B174A (heat pump controller communication fault) and DTC B174B (PTC heater status abnormal). Checked the heat pump controller power supply — fuse SB15 was intact and 12V voltage stable — and measured the heat pump controller CAN line resistance at 62Ω (termination resistor normal). Disassembly revealed condensation corrosion on the internal circuit board, caused by the vehicle having previously waded through deep water. **Resolution:** Replaced the heat pump air conditioning controller assembly (part number: HA-8112010) and the air conditioning system desiccant. Evacuated the AC lines and performed a pressure hold leak test, then recharged with 650g R134a refrigerant and 150ml POE compressor oil. After programming and matching the new controller, the fault was resolved.
Original source ↗
BYD DTC AI Analysis

BYD Han EV air conditioning panel unresponsive

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Symptoms: 2021 Han EV flagship, 42,000 km. The climate control panel intermittently goes black and the physical buttons become unresponsive, but voice control of the air conditioning remains functional. Fault frequency escalated from once a week to multiple times a day. Diagnosis: A scan tool retrieved DTCs B174A (HVAC control module communication fault) and B1A45 (abnormal panel button signal). Power and ground at the HVAC control module were normal. LIN bus voltage measured 10.8 V (normal range 9–11 V). With the HVAC control module connector disconnected, LIN-to-ground resistance measured 0 Ω (abnormal – short circuit). A section-by-section inspection of the harness revealed that the left-hand dashboard wiring clip had worn through the insulation, causing the LIN wire to short to body ground. Resolution: Repaired the chafed LIN wire, re-wrapped and rerouted the harness to prevent interference. Replaced the left-hand dashboard harness mounting bracket. Cleared the fault codes and tested repeatedly; panel response returned to normal and the communication fault did not recur.
Original source ↗
BYD DTC AI Analysis

BYD Tang DM rear air conditioning not working

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Symptoms: 2019 Tang DM owner reports front AC works normally, but rear independent AC (second-row overhead vents) blows no air. Rear AC panel controls unresponsive. Rear display shows "System Initialising" then goes black. Diagnosis: Scan tool detected DTCs B174A (Rear AC Control Module Communication Lost) and U0155 (Lost Communication With Instrument Panel Control Module). The rear AC control module fuse (RF16 10A) was blown; replacement fuse blew again, indicating a short. Removed and inspected the module: internal step-down chip burnt, module power pin shorted to ground. Root cause: when installing a dash cam, the power wire was incorrectly connected to the rear AC CAN lines, causing overvoltage damage. Resolution: Replaced rear AC control module assembly (part no. MEF-8112010B). Removed the incorrectly installed dash cam wiring and restored the original harness. Reset and adapted the front AC control module to match front-to-rear communication parameters. After repair, front and rear AC functions returned to normal, communication stable.
Original source ↗
BYD DTC AI Analysis

BYD Dolphin air conditioning cuts out intermittently

✓
Symptom Description: 2023 Dolphin Fashion Edition, six months old. The air conditioning would stop automatically after 15–20 minutes of operation, with no airflow from the vents. It would recover automatically after 5–10 minutes. When the fault occurred, the scan tool could read but could not clear the DTCs. Diagnostic Process: Retrieved DTCs B174A (Air Conditioning Control Module LAN Communication Fault) and U0245 (Lost Communication with Rear Spoiler Control Module, intermittent). Checked the software versions of the gateway module and air conditioning control module. Found the air conditioning control module was running early software (V1.8.2) with known communication protocol compatibility issues. Checked the CAN bus waveform and found abnormal spike interference during data transmission. Solution: Performed the technical upgrade: Upgraded the air conditioning control module software to V2.1.0 and the gateway module (GW) software to V3.5.1. After the upgrade, cleared all stored DTCs and ran the air conditioning system self-learning and a CAN bus communication test. Monitored for two weeks; the fault did not recur. Confirmed the fault was a communication anomaly caused by a software version defect.
Original source ↗
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.