AU
B2A2712

This DTC indicates the signal circuit of the automatic air conditioning system’s sun sensor (solar intensity sensor) has shorted to the vehicle power supply (B+, usually a constant 12V supply) — Seal U

Thermal Management System

This DTC indicates the signal circuit of the automatic air conditioning system’s sun sensor (solar intensity sensor) has shorted to the vehicle power supply (B+, usually a constant 12V supply).

The sun sensor generally uses a photodiode or photoresistor.

During normal operation, the sensor outputs an analog voltage signal of 0.1-4.9V to the air conditioning controller (integrated into the thermal management module).

The signal voltage increases as sunlight intensity increases.

When the signal wire shorts to power, the controller detects the voltage remains continuously above 4.9V (close to battery voltage, 12-14V) and logs a short-to-power fault.

This fault disables the automatic air conditioning system's solar compensation function.

The air conditioning controller cannot automatically adjust outlet air temperature and blower speed based on sunlight intensity.

In extreme cases, the short-circuit current may burn out the air conditioning controller's internal sampling circuit.

Consequently, the system reports a severe fault and may enter protection mode.

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Cases Logged
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Causes
Likely Causes — Seal U
  • 1The sunlight sensor wiring harness rubs against a sharp metal edge inside the dashboard. Damaged insulation causes the signal wire to short-circuit to the constant power wire. This commonly occurs after driving on rough roads or removing and installing the dashboard.
  • 2During front windshield replacement or dash cam installation, the technician accidentally damaged the sun sensor wiring harness, crushing the signal and power wires together and causing a short circuit.
  • 3Photoelectric element breakdown or seal failure inside the sun sensor causes an internal short circuit between the signal terminal and the power terminal. This typically occurs in high-temperature, high-humidity environments or after driving through water.
  • 4Internal sampling circuit fault in the air conditioning controller (thermal management ECU) abnormally pulling the signal line up to supply voltage, or water ingress into the controller connector causing a short circuit between terminals.
  • 5After a minor impact in the front or passenger compartment, the wiring harness bracket deforms and shifts, causing the sun sensor wiring harness to interfere with and chafe against constant power circuits such as the cigarette lighter and reading lamp.
What To Do
  • 1
    Use the VDS2100 diagnostic tool to read the DTC freeze frame, confirm the sunlight sensor voltage value when the fault occurred (typically reading above 5.0V or 12V), and observe whether the current data stream remains abnormal.
  • 2
    Remove the upper instrument panel trim. Locate the sunlight sensor below the windscreen (usually integrated into the same module as the automatic headlight sensor) and disconnect the sensor connector.
  • 3
    Measure the voltage between the sensor connector signal pin (usually pin 2) and ground: if the voltage remains 12 V, this indicates a short circuit in the wiring harness; if the voltage is 0 V, this indicates an internal short circuit in the sensor.
  • 4
    If the wiring harness has a short circuit, inspect the sensor wiring harness routing up to the air conditioning controller (located inside the dashboard or front compartment). Pay special attention to wear or damaged insulation at the dashboard frame pass-through hole and the firewall rubber grommet. Repair and re-wrap the damaged wiring harness.
  • 5
    If the sensor has an internal short circuit, replace the sunload sensor assembly (part number usually has a BC series prefix). After replacement, use a diagnostic tool to perform A/C system self-learning or sensor calibration (required on some models).
  • 6
    Clear the fault code. Read the data stream under both bright flashlight illumination and shaded conditions. Confirm the voltage changes normally within the 0.5V-4.5V range. Road test the vehicle to verify the automatic air conditioning solar compensation function is restored.
Real-World Cases
BYD DTC AI Analysis

Song MAX aftermarket dashboard modification caused wiring harness chafing and short circuit

✓
A 2018 Song MAX DM lost air conditioning cooling and set code B2A2712 after a head-up display (HUD) installation. Technicians found the aftermarket shop had failed to reposition the sunlight sensor harness when removing the dashboard, trapping it against the frame. While driving, the harness chafed against the metal edge, wearing through the insulation and shorting the signal wire to constant power. Repair: Re-routed the harness, wrapped it in abrasion-resistant corrugated tubing, and replaced the damaged sunlight sensor connector. Fault resolved.
BYD DTC AI Analysis

Sensor inoperative after front windscreen replacement on Qin Pro EV

✓
2019 BYD Qin Pro EV: One week after the dealership replaced the front windscreen, the instrument cluster displayed 'Thermal Management System Fault' and stored DTC B2A2712. Technicians found the sunlight sensor harness pinched between the windscreen and dashboard from the glass installation, breaking internal copper conductors and shorting them to the power wire. The diagnostic scanner showed sensor voltage locked at 13.2V (battery voltage). Repaired the damaged harness, adjusted the routing to prevent further pinching, cleared the fault codes, and the system returned to normal.
BYD DTC AI Analysis

Water entered the sensor connector and caused a short circuit after driving through deep water.

✓
A 2017 Qin EV300 developed automatic air conditioning fan speed control failure after driving through water during heavy rain. Diagnosis found abnormally high signal voltage from the sunlight sensor. Removing the sensor revealed water staining and corrosion inside the connector. The sensor mounts below the windscreen; a blocked drain channel allowed water to backflow into the connector. Technicians cleaned and dried the sensor connector, cleared the front windscreen drain channel, and applied insulating silicone grease to the connector to prevent future water ingress. Fault resolved.
BYD DTC AI Analysis

Song MAX Thermal Management Controller Internal Fault Misdiagnosis

✓
2019 Song MAX repeatedly throwing DTC B2A2712. Replaced the sunload sensor and wiring harness; fault persisted. Deep inspection found a burnt pull-up resistor in the air conditioning controller (integrated thermal management module) internal signal sampling circuit, forcing the sensor signal line to 12V. Controller internal short-circuit current back-fed into the sensor circuit, causing the fault. Replaced the AC controller assembly (PN 6A-8112010), continuity-tested the wiring harness to confirm no short-circuit risks, and delivered the vehicle.
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.