AU
B2A5811

This DTC indicates a short to ground in the driver-side face vent (FACE mode) temperature sensor signal circuit — Seal U

Thermal Management System

This DTC indicates a short to ground in the driver-side face vent (FACE mode) temperature sensor signal circuit.

The sensor is an NTC (negative temperature coefficient) thermistor.

During normal operation, it sends a 0.5-4.5V analog voltage signal to the air conditioning control unit (ACU) to indicate the vent outlet temperature.

When the ACU detects the signal voltage remains below 0.1V (logic low threshold) for longer than the set time (typically 200-500ms), it determines a short to ground.

This fault causes the ACU to enter Limp Home Mode, disables independent dual-zone temperature control, and may lock the driver-side temperature flap in the full cold or full hot position, severely affecting air conditioning comfort.

In extreme cases, continuous full-load compressor operation affects overall vehicle energy consumption.

5
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Internal thermistor breakdown or encapsulation failure in the temperature sensor causes the signal terminal to short to the housing/shielding layer.
  • 2During removal and installation, retaining clips or sharp metal bracket edges chafe and damage the instrument panel internal wiring harness insulation, causing the signal wire (usually the cathode wire) to contact body ground.
  • 3Seal failure at the connector between the air conditioning controller and the sensor (usually located on the evaporator housing or instrument panel crossmember) allows water ingress, causing a short circuit between terminals or a short to the housing.
  • 4Breakdown of the sampling resistor or protection diode in the internal signal acquisition circuit of the Air Conditioning Control Unit (ACU) pulls the signal line low.
  • 5Improper wiring harness routing during repair caused the instrument panel frame to pinch the harness, resulting in an intermittent short to ground.
What To Do
  • 1
    Use VDS2000 or a dedicated BYD diagnostic tool to read complete DTCs and freeze frame data. Record the ambient temperature, air outlet temperature set value, and actual feedback value when the fault occurred. Confirm if B2A5811 is a current fault (Present).
  • 2
    Remove the trim panel below the driver side instrument panel and locate the face outlet temperature sensor (typically located on the evaporator housing outlet air duct, with a white or grey 2-PIN connector). Visually inspect the connector for looseness, water ingress, or ablation.
  • 3
    Disconnect the sensor connector and use a multimeter to measure the resistance between the two terminals on the sensor side. At 25°C, the resistance should be 1.6-1.8 kΩ (refer to the temperature-resistance curve table in the vehicle repair manual). If the resistance is close to 0 Ω, replace the sensor.
  • 4
    Leave the sensor connector disconnected. Measure the resistance between the harness-side signal wire (SIG) and body ground. Normal resistance is infinity (OL). If the resistance is less than 1Ω, a short to ground exists. Inspect the wiring for damage, focusing on the edges of the dashboard metal bracket and the harness retaining clips.
  • 5
    Measure the harness-side signal wire voltage: With the ignition switch ON and the A/C ON, the ACU normally supplies a 5V reference voltage. If the voltage is 0V and resistance to ground is normal, check the ACU connector. If the voltage is normal but the fault code sets after reconnecting the sensor, this confirms an internal short circuit in the sensor.
  • 6
    If the wiring harness and sensor are normal, measure circuit continuity and insulation between the corresponding ACU pin and the sensor connector. If necessary, use a jumper wire to verify the wiring harness.
  • 7
    After replacing the faulty component (sensor) or repairing the wiring harness, clear the DTC and perform the air conditioning system self-learning procedure (e.g., air flap initialization). Verify normal air outlet temperature changes and data stream feedback at different temperature settings (LO-HI).
Real-World Cases
BYD DTC AI Analysis

Song MAX temperature sensor wiring harness worn after dashboard removal and installation

✓
Vehicle: 2018 Song MAX 1.5T. Symptom: After removing and reinstalling the dashboard to replace the evaporator, the AC panel displayed ‘E1’. Scan tool read DTC B2A5811. Diagnosis: Sensor resistance was normal. Measured the harness-side signal wire: continuity to ground. Removed the dashboard again and found the harness pinched under the crossmember bracket during reassembly. The insulation had worn through, shorting the copper core to chassis. Repair: Repaired the damaged harness, wrapped it with abrasion-resistant corrugated tubing, and rerouted it away from sharp metal edges. Fault cleared.
BYD DTC AI Analysis

Water ingress and corrosion caused intermittent short circuit in Qin Pro EV sensor.

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Vehicle: 2019 Qin Pro EV. Symptoms: AC malfunctions after rain or car washing, fault light on; returns to normal in dry weather. Diagnosis: Read historical DTC B2A5811, intermittent. Found green copper corrosion inside the temperature sensor connector beneath the evaporator housing; sealing ring deteriorated. Water caused a short between the signal terminal and sensor metal housing. Solution: Cleaned connector terminals, replaced sensor and sealing ring, applied waterproof silicone grease to the connector. Fault has not reoccurred.
BYD DTC AI Analysis

Qin EV AC control module internal circuit damage

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Vehicle: 2017 Qin EV300. Fault: Air conditioning not cooling; no airflow from driver's side vents. Diagnosis: Retrieved DTCs B2A5811 and B2A5813 (open circuit) alternating. Measured sensors and wiring harness – normal. Testing ACU output found 5V reference voltage shorted to ground. Disassembled the ACU; filter capacitor in the internal sampling circuit had broken down. Solution: ACU is an integrated module. Replaced the assembly, programmed and matched. Fault resolved.
BYD DTC AI Analysis

Song MAX temperature sensor body deteriorated and shorted

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Vehicle: 2017 Song MAX, 80,000 km. Symptom: Auto climate mode inoperative, continuously blowing hot air. Diagnosis: Data stream showed driver-side face vent temperature sensor fixed at -40°C (default substitute value used by ACU when detecting short to ground). Measured resistance between sensor terminals: 0.2Ω, confirming internal thermistor short. Solution: Replaced driver-side face vent temperature sensor (Part No.: EG-8120410). After clearing DTCs, temperature display returned to normal and auto climate function restored.
BYD DTC AI Analysis

Aftermarket modification caused wiring harness short circuit

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Vehicle: 2019 BYD Qin Pro DM. Fault: AC fault code B2A5811 appeared after installing a 360-degree panoramic camera system. Diagnosis: The retrofit shop pierced the AC temperature sensor harness while fishing wires behind the dashboard with steel wire, causing the signal wire to short against the dashboard metal frame. Repair: Repaired the two damaged wires (signal and ground), insulated them with heat-shrink tubing, and rerouted the aftermarket harness to avoid the factory cable-tie locations.
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.