AU
B1108

For the BYD Song MAX (2017–2019), DTC B1108 indicates a short circuit in the PM2 — Seal U

Thermal Management System

For 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.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Water ingress or moisture inside the PM2.5 sensor causing a circuit board short circuit (commonly due to A/C evaporator condensate leakage or vehicle wading).
  • 2Worn sensor wiring harness insulation contacts sharp edges of the body metal frame or A/C housing, causing a short to ground (common if the harness is not properly routed after dashboard removal and installation).
  • 3Sensor connector pins bent, backed out, or oxidized due to water ingress, causing a short circuit between the power supply and signal pins.
  • 4Air conditioning controller internal signal sampling circuit fault, incorrectly identifying a normal sensor signal as a short circuit (false fault).
  • 5Improper wire splicing during non-OEM modifications (such as installing an air purifier or tapping power for a dashcam) shorts the power wire to the sensor signal wire.
What To Do
  • 1
    Use the BYD VDS or a dedicated diagnostic tool to read the DTC freeze frame, confirm the ambient temperature, humidity, and vehicle status at the time of the fault, and determine if the fault is current or historical.
  • 2
    Remove the PM2.5 sensor located on the right side of the dashboard or the upper section of the air conditioning unit (on the Song MAX, usually behind the glove box or inside the center console). Visually inspect the connector for water stains, oxidation, or deformed pins.
  • 3
    Disconnect the sensor connector. On the sensor body, use a multimeter to measure the resistance between the power supply and ground, and between the signal and ground. If the resistance is less than 1 Ω or close to 0 Ω, the sensor has an internal short circuit. Replace the sensor.
  • 4
    Keep the sensor disconnected and measure the harness-side connector: power pin to ground resistance should be infinite (OL), and signal pin to ground resistance should be a specific value (refer to the workshop manual, usually >1kΩ). If a short circuit exists, inspect the wiring harness.
  • 5
    Follow the sensor wiring harness routing (via the instrument panel frame and air conditioning housing to the HVAC ECU) and inspect the protective sleeve integrity. Focus on areas contacting metal edges. Repair and rewrap any damaged wiring harness.
  • 6
    If the wiring harness is normal, use an oscilloscope to check the signal wire waveform: it should be a pulse signal or varying voltage. A fixed supply voltage indicates a possible short circuit in the ECU internal pull-up resistor. Repair or replace the air conditioning controller.
  • 7
    After replacing the sensor, clear the fault code. Use a smoke generator or incense to test for changes in the sensor data stream. Confirm the air quality value rises normally with smoke concentration and the system automatically switches between fresh air and recirculation modes.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Dashboard wiring harness chafing caused PM2.5 sensor short circuit in 2018 Song MAX

✓
The vehicle came in with DTC B1108 and the auto air-conditioning mode had failed. Technicians found 0V at the PM2.5 sensor connector (normally 5V reference). Disconnecting the sensor and measuring the harness revealed the power wire shorted to chassis. Removing the dashboard showed the wiring harness had been pinched by a metal bracket during an aftermarket navigation installation at an independent workshop. Long-term vibration damaged the insulation, exposing bare copper wires that shorted to the chassis. They repaired the damaged wiring, rerouted the harness and added abrasion protection sleeves. Fault resolved.
BYD DTC AI AnalysisFrom Chinese market (translated)

2019 Song MAX evaporator leak caused sensor water ingress and short circuit

✓
Owner reported poor A/C cooling performance. VDS retrieved DTCs B1108 and B2A2F (evaporator temperature abnormal). Inspection found obvious water staining inside the PM2.5 sensor connector; internal resistance measured 0.2Ω (internal short). Further inspection found a blocked evaporator drain hose, causing condensate to overflow into the sensor mounting position. Cleared the drain hose, replaced the PM2.5 sensor, and cleaned and dried the wiring harness connector. The fault has not reoccurred.
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin EV Air Conditioning Fault B1108 (Sun Sensor Failure Case Reference)

✓
Note: This case applies to Qin EV and similar models. On these vehicles, B1108 is defined as a sun sensor fault. The automatic climate control could not regulate outlet temperature in auto mode, and the diagnostic tool displayed B1108. Checked the sun sensor wiring (located under the windscreen at the centre of the dashboard) and found the dashboard wiring harness had worn, causing an intermittent short and irregular signal waveform. Repaired the wiring insulation and replaced the sun sensor, restoring normal automatic climate control operation. For Song MAX models with similar symptoms, check the PM2.5 sensor wiring.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

Song DM intermittently throwing B1108/B1109 fault codes (reference case)

✓
Note: Applies to Song DM and similar models. After the vehicle sat in direct sun, the A/C failed and set codes B1108 and B1109. Found the sunlight sensor blocked by a dashcam sunshade pad; its connector pins were oxidised. Cleaned the oxidation and removed the obstruction, but the fault remained. Replaced the sensor to fix it. For B1108 on the Song MAX, check the PM2.5 sensor location for blockages (e.g., cabin filter debris) and connector oxidation.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

E5 air conditioning controller wiring harness high resistance fault (reference case)

✓
Note: Applies to E5 and similar models. Air conditioning works intermittently, reporting B1108 communication fault. Measured resistance between sun sensor and controller signal line: 50Ω (normal <1Ω). Harness looked intact, but most internal copper strands were broken. Repaired the broken harness, fault cleared. For Song MAX with B1108, run a continuity test even if the harness looks fine—internal breaks can cause abnormal signals.
Original source ↗
Other Seal U Fault Codes
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.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]