AU
B1109

DTC B1109 indicates an open circuit in the communication line between the air conditioning system PM2 — Seal U

Thermal Management System

DTC 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.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1PM2.5 sensor internal open circuit (due to sensor aging, internal component damage, or corrosion from water ingress)
  • 2Loose sensor wiring harness connector, backed-out pins, terminal corrosion, or poor contact (common after dashboard removal/installation or vehicle wading)
  • 3Open sensor power supply circuit or blown fuse (B+ or IGN power supply circuit interrupted)
  • 4Sensor signal circuit (LIN bus or analog signal wire) open, chafed, or short to ground (wiring harness chafes against the instrument panel frame over time, causing insulation damage)
  • 5Air conditioning control unit (AC ECU) internal signal receiving interface circuit fault (ECU internal cold solder joint or damaged chip)
What To Do
  • 1
    Use a BYD dedicated diagnostic tool (VDS or ED400) to read the fault code. Confirm B1109 is active. Record the freeze frame data, clear the fault code, and check for an intermittent fault.
  • 2
    Visually inspect the PM2.5 sensor installation (located near the air conditioning filter element or inside the right dashboard air duct). Check the sensor connector for looseness, backed-out pins, or water ingress, and check for foreign objects blocking the sensor air inlet.
  • 3
    Measure sensor supply voltage: Turn the ignition switch to ON, disconnect the sensor connector, and measure the voltage between the power supply terminal (usually PIN1) and ground. Standard value: 12V±0.5V (or 5V depending on the model). If the voltage is abnormal, check the dashboard distribution box fuse and power supply circuit continuity.
  • 4
    Measure the sensor ground circuit: Use a multimeter to measure the resistance between the sensor ground terminal (usually PIN3 or PIN4) and body ground. The standard value must be less than 1Ω. If the resistance is too high, check for a loose ground point or an open circuit in the wiring harness.
  • 5
    Measure signal line continuity: Disconnect the AC ECU and sensor connectors. Measure the harness resistance between the sensor signal terminal (LIN line is usually PIN 2) and the corresponding AC ECU terminal. Resistance should be less than 1 Ω. Check the insulation of this line to ground and power supply. Resistance should be greater than 10 MΩ.
  • 6
    If all circuit measurements are normal, replace the PM2.5 sensor assembly. After installation, clear the fault code and use the diagnostic tool to perform 'Air Quality Sensor Calibration' (if required). Spray smoke into the air inlet to observe sensor data stream changes and confirm fault resolution.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Song MAX A/C reports code B1109 after dashboard rattle repair

✓
A 2018 Song MAX came in with the owner reporting that after the dashboard was removed and refitted to fix a rattle, the air conditioning auto mode stopped working and the instrument cluster displayed an air quality detection fault. A VDS scan revealed current fault code B1109. Inspection showed the technician had failed to fully seat the PM2.5 sensor connector during the dashboard work, causing a loose connection at the sensor power terminal. After reseating the sensor connector and wrapping it with waterproof tape, the live data showed the PM2.5 readings returned to normal and the fault cleared.
BYD DTC AI AnalysisFrom Chinese market (translated)

After driving through water, the Song MAX air conditioning system logged DTC B1109 and the air purification failed.

✓
A 2019 Song MAX had water ingress inside the dashboard due to a blocked windscreen scuttle drain. The climate control panel displayed an 'Air Quality Sensor Fault' warning. A scan tool reported DTC B1109. Removed the PM2.5 sensor at the HVAC air intake for inspection and found internal water damage and corrosion, with an open circuit on the circuit board. Power supply and earth tested normal, and the signal wire showed continuity. Replaced the PM2.5 sensor (Part No.: BYD-EP-3742010) and cleared the scuttle drain, fully eliminating the fault.
BYD DTC AI AnalysisFrom Chinese market (translated)

Song MAX intermittently reports B1109 fault when driving on bumpy roads.

✓
2017 Song MAX. The owner reported that when driving on rough roads, the air conditioning occasionally switched to recirculation mode and the warning light illuminated, though operation was normal on smooth surfaces. The diagnostic tool displayed intermittent fault code B1109. Inspection found the sensor wiring harness improperly secured at the dashboard metal bracket; prolonged vibration had worn through the insulation and partially fractured the internal copper wires, creating an intermittent connection. Cut out the damaged section, resoldered the wires, and insulated them with heat-shrink tubing. Repositioned the harness clip to prevent contact with metal edges. The fault has not recurred.
BYD DTC AI AnalysisFrom Chinese market (translated)

After driving through water, the Song MAX air conditioning system logged DTC B1109 and the air purification failed.

✓
Rainwater leaked into the dashboard of a 2019 Song MAX after the front windscreen drain channel blocked. The climate control panel displayed 'air quality detection fault' and the scan tool reported DTC B1109. Inspection found water damage inside the PM2.5 sensor at the AC air intake, corroding the circuit board and creating an open circuit. Power supply and ground tested normal, and the signal line had continuity. Replaced the PM2.5 sensor (part number: BYD-EP-3742010) and cleaned the drain channel. Fault completely resolved.
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.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.B110A02DTC B110A02 indicates the thermal management master control unit (HVAC ECU) or body control module (BCM) failed to receive valid CAN data frames from the PM2.5 air quality sensor over consecutive communication cycles (typically 500ms per cycle). The sensor typically mounts at the HVAC unit air inlet or behind the glove box and transmits real-time particulate concentration data (unit: μg/m³) to the HVAC controller via the CAN bus. The fault is essentially a communication link layer timeout. Potential causes include physical layer wiring faults, sensor power supply failures, sensor internal MCU lockup, or CAN network topology damage. Although this non-critical fault does not directly affect driving safety, it disables the intelligent air purification strategy and the automatic fresh air/recirculation switching function. In extreme cases, it triggers a thermal management system degraded mode, limiting compressor power or fixing the air flap position.
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. Sources: [1]