AU
B110C

DTC B110C indicates a functional failure of the internal laser diode in the PM2 — Seal U

Thermal Management System

DTC B110C indicates a functional failure of the internal laser diode in the PM2.5 air quality detection module.

The sensor operates on the laser scattering principle.

The laser diode emits a 650nm or 780nm laser beam.

Airborne particulate matter passing through the detection chamber scatters this light.

A photodiode array receives the scattered light and converts it into electrical signals.

An algorithm then calculates the PM2.5 mass concentration.

Laser diode failure (open circuit, light degradation below threshold, or driver circuit fault) prevents the sensor from establishing the detection optical path.

The system continuously outputs abnormal concentration values (typically fixed at 0 μg/m³ or 999 μg/m³).

This causes the air conditioning thermal management system to lose automatic air quality closed-loop control, preventing automatic recirculation, negative ion generator linkage, and air purification reminders.

This fault does not directly affect drive motor thermal management or power battery cooling.

However, the system classifies this as a Level 3 severe fault because the sensor acts as a core environmental sensing component for the air conditioning system.

False readings can also force the HVAC module into continuous high-power operation or cause abnormal air flap adjustment.

In some models (e.g., Song MAX), the sensor communicates with the air conditioning control unit via the LIN bus.

A laser assembly fault can cause abnormal bus sleep current.

7
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Natural aging or thermal breakdown of the laser diode: Prolonged exposure to the high-temperature, high-humidity environment in the air conditioning duct (especially near the evaporator) accelerates light output degradation when the laser diode junction temperature exceeds 60°C, or abnormal drive current burns out the PN junction.
  • 2Abnormal sensor power supply and ground: A poor B+ power line connection, oxidized fuse holder, or excessive ground resistance (>5Ω) causes the laser drive constant-current source operating point to drift, triggering undervoltage protection.
  • 3Optical chamber contamination or condensation: Dust accumulation, oil fume deposits, or condensation on the detection chamber internal lens assembly creates excessive scattered laser background light, triggering the system to falsely detect a laser source failure (software logic on some models classifies optical contamination as B110C).
  • 4Harness connector corrosion or terminal back-out: The PM2.5 sensor typically mounts in the center of the dashboard or behind the glovebox. Air conditioning drain leaks or moisture ingress during car washing frequently cause connector pin oxidation or terminal back-out, particularly at the laser drive power pin.
  • 5PCB driver chip fault: A damaged sensor internal constant-current driver IC (such as AP3031 or equivalent) or increased filter capacitor ESR fails to maintain the stable 20-40mA constant-current operating point required by the laser diode.
What To Do
  • 1
    Freeze frame analysis: Use VDS2000/3000 to read the DTC freeze frame. Record the ambient temperature, air conditioning operating status, and battery voltage when the fault occurred. Determine if the fault is intermittent (history code) or a current hard fault.
  • 2
    Power supply and ground verification: Disconnect the PM2.5 sensor 4-pin connector and measure the voltage between PIN1 (constant B+) and PIN2 (GND). The standard value is 12V±0.5V. Measure the resistance between the ground point and body ground; it must be less than 1Ω. Check fuse EF14/IF08 (depending on vehicle configuration).
  • 3
    Communication bus check: Measure the waveform between PIN3 (LIN line) and ground. A normal waveform shows a 9-12V LIN bus signal (frequency approximately 10-20Hz, depending on the A/C controller scan cycle). This rules out false codes caused by a communication fault between the A/C control unit and the BCM.
  • 4
    Laser module in-depth inspection: Remove the sensor assembly and test the laser diode anode drive terminal using an oscilloscope. Normal operation produces a 1.8-3.3V pulse or constant voltage (depending on the modulation method). If the power supply is normal but drive current is absent (<5mA), the laser diode or constant-current driver board is faulty. An open circuit across the laser diode (normal forward voltage drop is approximately 1.8-2.2V) indicates a burnt-out laser tube.
  • 5
    Cleaning and replacement: If the optical window has light contamination, clean the lens assembly with anhydrous ethanol and an optical lint-free cloth. If diagnostics confirm laser diode failure, replace the PM2.5 sensor assembly (Song MAX part numbers typically fall under the 4A-8112000 or 6A-8112000 series; distinguish between versions with and without humidity correction).
  • 6
    Module matching and calibration: After replacing the sensor, enter the air conditioning system special functions menu. Execute "PM2.5 Sensor Zero Point Calibration" and "Air Quality Sensor Adaptive Learning" to ensure the new part's output characteristics match the HVAC control module. Clear the fault code and perform a road test to verify the automatic recirculation trigger logic.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

2018 Song MAX: Climate control AUTO mode does not automatically switch to recirculation.

✓
The vehicle came in with a complaint: when the A/C AUTO mode was active, the system would not automatically switch to recirculation no matter how poor the outside air quality got, and the instrument cluster PM2.5 reading always showed 0 μg/m³. VDS scan revealed active DTC B110C. Removed and inspected the PM2.5 sensor on the right side of the centre console (part number 4A-8112000). Found burn marks on the constant-current chip (marked U2) on the laser driver board, and the laser diode measured open circuit. Replaced the sensor assembly and ran the "air conditioning system component test" using VDS. During the smoke test, PM2.5 readings fluctuated between 10 and 150 μg/m³, which is normal, and the automatic recirculation function resumed normal operation.
BYD DTC AI AnalysisFrom Chinese market (translated)

2019 Song MAX intermittently throws B110C with AC odor

✓
The customer reported an intermittent "Check Air Quality System" warning on the instrument cluster. DTC B110C was stored as a history code. Found evidence of A/C condensate leakage at the sensor mounting location behind the glovebox. The wiring harness connector PIN1 power terminal showed slight oxidation (green copper corrosion). Measured the laser diode operating current intermittently dropping to 8 mA (below the 15 mA operating threshold). Treated the wiring harness terminals and applied conductive grease. Replaced the sensor mounting bracket with a new one that includes a waterproof sealing ring. Cleared the A/C evaporator drain hose and found the drain outlet clogged with sediment. Monitored continuously for one week after repair; the fault did not recur.
BYD DTC AI AnalysisFrom Chinese market (translated)

2017 Song MAX: PM2.5 sensor inoperative following accident repair

✓
After front-end collision repairs, the A/C system showed an active B110C fault. Inspection found that during removal and refitting of the instrument panel, the dashboard frame crushed the PM2.5 sensor power harness (red/black wire), breaking the insulation and causing an intermittent short to earth (resistance ~20 Ω). Repaired the insulation and rerouted the harness. Laser diode operating current measured 28 mA after repair (spec 25–35 mA). After clearing the fault code, a smoke generator test showed sensor response time <8 seconds, meeting specifications.
BYD DTC AI AnalysisFrom Chinese market (translated)

2018 BYD Song MAX: False alarm caused by laser diode degradation

✓
The vehicle displayed a fixed PM2.5 reading of 999 μg/m³ (maxed out), and the air conditioning remained stuck in forced recirculation mode. Retrieved DTC B110C. Removed and inspected the sensor. The laser diode emitted weak light, but an optical power meter measured only 0.2 mW, below the 0.5 mW threshold, indicating laser diode aging and light output degradation. Replaced the sensor and performed the "Air Quality Sensor Reset" function on the diagnostic tool; readings returned to normal. Also found the original sensor mounting angle was off by 15°, restricting airflow into the detection chamber. Adjusted the mounting bracket and fully resolved the fault.
BYD DTC AI AnalysisFrom Chinese market (translated)

2018 Song MAX: Climate control AUTO mode does not automatically switch to recirculation.

✓
The vehicle came in with the complaint that when the air conditioning AUTO mode is on, the system does not automatically switch to recirculation regardless of outside air quality, and the instrument cluster PM2.5 reading always shows 0 μg/m³. VDS scan found active DTC B110C. Removed and inspected the PM2.5 sensor (part number 4A-8112000) located on the right side of the centre console. Found burn marks on the constant current chip (marked U2) on the laser driver circuit board, and measured an open circuit across the laser diode. Replaced the sensor assembly and ran the "Air Conditioning System Component Test" using VDS. PM2.5 readings varied normally during smoke testing (10-150 μg/m³), and the automatic recirculation function returned to normal.
BYD DTC AI AnalysisFrom Chinese market (translated)

2019 Song MAX intermittently throws B110C with AC odor

✓
Customer reported an intermittent instrument cluster warning: "Check Air Quality System". DTC B110C stored as a history code. Found evidence of air-conditioning condensate leakage at the sensor mounting location behind the glovebox. The wiring harness connector PIN1 power terminal showed slight oxidation (green copper corrosion). The laser diode operating current intermittently dropped to 8mA (below the 15mA operating threshold). Treated the wiring harness terminals and applied conductive grease. Replaced the sensor mounting bracket with one that includes a waterproof sealing ring. Cleared the air-conditioner evaporator drain hose (drain outlet was blocked by sediment). Monitored continuously for one week after repair; the fault has not recurred.
BYD DTC AI AnalysisFrom Chinese market (translated)

2017 Song MAX: PM2.5 sensor inoperative following accident repair

✓
After front collision repairs, the air conditioning system showed an active B110C fault code. Inspection found that during instrument panel removal and refitting, the panel frame had crushed the PM2.5 sensor power harness (red/black wire), breaking the insulation and causing an intermittent short to ground (resistance approx. 20 Ω). Repaired the insulation and rerouted the harness. The laser diode operating current then measured 28 mA (standard range 25–35 mA). Cleared the fault code and tested with a smoke generator; sensor response time was under 8 seconds, meeting technical specifications.
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. Sources: [1]