AU
B2A5911

This DTC indicates a short to chassis ground (GND) in the signal circuit of the driver-side Foot Outlet Temperature Sensor — Atto 8

Thermal Management System

This DTC indicates a short to chassis ground (GND) in the signal circuit of the driver-side Foot Outlet Temperature Sensor.

In the BYD thermal management system, this NTC thermistor sensor monitors the actual temperature at the foot air duct outlet.

The signal voltage is typically a 0-5V analog signal.

A short to ground causes the ECU to continuously read an abnormally low voltage near 0V, registering a corresponding temperature of -40℃ or an extreme low.

This forces the air conditioning control unit into fault protection mode.

The system may force the compressor off, switch to demist mode, or restrict battery coolant flow.

These conditions severely impact occupant comfort and battery/motor thermal management efficiency.

In extreme cases, this triggers high-voltage system overheat protection.

5
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1A breakdown and short circuit of the thermistor inside the sensor body causes continuity between the signal and ground terminals.
  • 2Vibration wears through the wiring harness insulation where it passes through the dashboard or floor, causing the signal wire (usually yellow/green or blue/white) to contact the metal body frame.
  • 3Condensation leakage inside the A/C duct causes a short circuit or corrosion between sensor connector pins (typically located on the duct housing).
  • 4Metal components such as seat rails or pedal brackets pinch or cut the floor wiring harness after adjustment, causing the signal wire to short to ground.
  • 5Internal signal processing circuit fault in the air conditioning control module (AC ECU) causes an abnormal drop in input impedance, resulting in a short to ground.
What To Do
  • 1
    Use the VDS2000/3000 diagnostic tool to read the complete fault code stream, confirm B2A5911 is a current fault (Present) rather than a history fault, and check if 'Driver footwell temperature' in the data stream displays -40°C, 0°C, or a fixed extremely low value.
  • 2
    Remove the driver-side footwell air outlet trim panel and locate the temperature sensor (usually on the lower surface of the air duct, 2-pin connector). Inspect the connector for looseness, water ingress, or obvious corrosion. Measure the sensor resistance (normal is approx. 1.8-2.2 kΩ at 25°C; close to 0 Ω when shorted to ground).
  • 3
    Disconnect the sensor connector. Use a multimeter to measure the resistance between the harness-side signal pin (B+) and body ground. If the resistance is less than 5Ω, the wiring harness is shorted to ground. If the resistance is infinite, the sensor is faulty.
  • 4
    If the wiring harness is shorted, visually inspect the harness routing under the instrument panel and along the floor. Focus on the harness sheath integrity where it passes through the firewall, near the seat rails, and around the pedal bracket. Look for wear, crushing, or damaged insulation.
  • 5
    Repair the wiring harness: Insulate the damaged section with heat-shrink tubing or waterproof tape. Re-secure the harness to prevent contact with sharp metal edges. If the sensor is faulty, replace it with a genuine temperature sensor (usually identified as '温度传感器' or 'Foot Temp Sensor').
  • 6
    After reconnecting, use the diagnostic tool to perform 'A/C system self-learning' or 'temperature sensor calibration' (if supported by the menu). Clear the fault code and observe the data stream to verify the temperature changes normally during A/C operation (typical range: -30°C to +60°C).
  • 7
    Perform a road test or static test to verify the fault code does not recur. Confirm the footwell air outlet temperature control operates normally and no abnormal protection mode triggers.
Real-World Cases
BYD DTC AI Analysis

Song MAX seat rails crushed wiring harness, causing intermittent short circuit

✓
A 2018 Song MAX DM repeatedly set DTC B2A5911, with the warning light appearing intermittently. The fault consistently reproduced when adjusting the driver's seat to the rearmost position. Removing the seat revealed the floor wiring harness pinched at a seat rail mounting bolt; damaged insulation caused an intermittent short to ground. Rerouted the harness and secured it with dedicated wiring clips, positioning it clear of the seat track travel. Fault eliminated.
BYD DTC AI Analysis

Qin EV sensor body thermistor aging breakdown

✓
2017 Qin EV300 with poor AC cooling and DTC B2A5911. Sensor resistance measured only 12Ω (normal approx 2kΩ), confirming internal thermistor short. Replaced the driver's footwell temperature sensor (located at the lower section of the HVAC unit; requires removal of the lower glovebox trim panel). Live data shows normal temperature and automatic climate control restored.
BYD DTC AI Analysis

Qin Pro DM AC duct poor sealing causing condensate corrosion

✓
The 2019 Qin Pro DM frequently set DTC B2A5911 during the rainy season. Inspection found the evaporator housing seal had perished, allowing condensate to leak into the sensor mounting hole and oxidise and short the connector pins. Replaced the evaporator housing gasket, cleaned the connector terminals and applied conductive grease, and relocated the sensor to a higher position to avoid water pooling.
BYD DTC AI Analysis

Song MAX instrument panel wiring harness chafing against bracket

✓
A 2019 Song MAX with 80,000 km set DTC B2A5911 with no warm air at the driver's footwell. Removed the lower dashboard trim and found the temperature sensor wiring harness chafed through at the steering column mounting bracket from prolonged vibration, exposing copper conductors contacting the bracket. Repaired the harness, added a rubber protective sleeve and cable ties at the interference point, and ensured >10 mm clearance from metal components.
BYD DTC AI Analysis

Misdiagnosis of internal circuit fault in Qin PRO EV AC ECU

✓
After replacing the battery pack on a 2019 Qin Pro EV, DTC B2A5911 set, but the sensor and wiring harness tested normal. Further inspection revealed the AC control module (located in the centre of the dashboard) was crushed during removal and installation, damaging the internal signal acquisition circuit and causing a short to ground. Replacing the AC control module and reconfiguring the thermal management parameters resolved the fault.
Other Atto 8 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.