AU
B122B11

In DTC B122B11, "B" designates the Body control system, "122B" identifies the coolant temperature sensor, and "11" indicates a signal circuit short to ground per the BYD diagnostic protocol — Atto 8

Thermal Management System

In DTC B122B11, "B" designates the Body control system, "122B" identifies the coolant temperature sensor, and "11" indicates a signal circuit short to ground per the BYD diagnostic protocol.

The sensor mounts at the PTC heater outlet or in the motor cooling circuit.

It utilizes an NTC (negative temperature coefficient) thermistor, where resistance decreases as temperature increases.

The ECU sets this code when the sensor signal voltage remains below 0.1V, indicating a direct ground.

A short circuit causes the ECU to read an extremely high temperature, as low NTC resistance corresponds to high heat.

The thermal management system immediately enters protection mode.

It forces the PTC heater off (disabling cabin heating) and may limit motor power to prevent overheating, classifying this as a Level 3 severe fault.

This fault links to the air conditioning system because the PTC serves as the core EV cabin heating component, placing its temperature monitoring under the A/C thermal management subsystem.

5
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1Internal short circuit in the PTC heater water outlet temperature sensor body: Internal thermistor encapsulation failure or damaged lead wire insulation shorts the signal terminal to the grounded housing.
  • 2Wiring harness wear causing short to ground: Vibration and chafing damage the sensor wiring harness insulation in the engine or front compartment (especially near the PTC heater high-temperature area or harness retaining clips), grounding it to the vehicle body metal.
  • 3Connector water ingress and corrosion: An aged sealing ring on the front compartment low-voltage connector (usually located near the PTC assembly or front compartment power distribution box) allows water to enter after washing the vehicle or wading, causing a short circuit between the signal pin and the ground pin.
  • 4Improper repair work: Failing to correctly secure the wiring harness during previous PTC or air conditioning system repairs caused it to rub against a sharp-edged metal bracket, or failing to fully seat the connector caused the terminal to back out and contact the housing.
  • 5Internal fault in the PTC heater assembly: Short circuit in the temperature sampling circuit on the PTC internal control board, or high PTC housing temperature causing the sensor wiring harness insulation to melt and fuse.
What To Do
  • 1
    Diagnostic tool verification: Use VDS or a dedicated BYD diagnostic tool to read the complete fault codes and freeze frame data. Verify if the coolant temperature reading at the time of the fault shows an abnormal value (such as -40°C or above 140°C). Record the ambient temperature for comparison.
  • 2
    Visual inspection: In the front compartment, check the temperature sensor connector at the PTC heater water outlet (black high-voltage component located on the left or right side) for looseness, water ingress, or burn marks. Check if the wiring harness retaining clip is detached.
  • 3
    Disconnect and measure: Disconnect the sensor connector. Measure the resistance between the two sensor terminals (approx. 2.5kΩ at 20°C). If the resistance is close to 0Ω, replace the sensor. If normal, measure the resistance to ground at the harness-side signal pin.
  • 4
    Harness continuity test: Disconnect the thermal management controller (or air conditioning controller) connector. Measure the resistance to ground of the sensor signal wire (usually yellow/green or brown/white). Resistance must be greater than 1MΩ. If continuity exists, trace the harness to locate the damage. Focus inspection on the PTC bracket edge and firewall pass-through.
  • 5
    Voltage check: Reconnect the sensor, turn the ignition switch to ON, and measure the voltage between the signal wire and ground. Normal voltage is 2.5-3.5V (varies with temperature). If the voltage is 0V and the wiring harness is not shorted, check the ECU terminal or replace the thermal management controller.
  • 6
    Repair and Verification: Repair the damaged wiring harness (use heat-shrink tubing for waterproofing) or replace the sensor. Route the wiring harness away from high-temperature components (PTC housing surface temperature can exceed 80°C). Secure the harness with cable ties, leaving sufficient slack. Clear the fault code and run the air conditioning in heating mode for 10 minutes to confirm the fault does not recur.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin EV300 PTC coolant temperature sensor wiring harness chafed and shorted

✓
The instrument cluster displayed a 'Motor coolant temperature too high' warning light, and the air conditioning had no warm air. The diagnostic tool read codes B122B11 and B123098 (left radiator fin overheating). Inspection found the retaining clip for the PTC heater outlet sensor wiring harness at the right-hand front compartment bracket was broken. The loose harness had chafed against the metal bracket over time, wearing through the insulation and exposing the copper core, which shorted to the bracket (ground). Repair: Repaired the damaged harness by wrapping it with waterproof tape and heat-shrink tubing. Rerouted and secured the harness with cable ties, keeping it clear of sharp edges. Replaced the broken retaining clip. Fault resolved.
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin Pro DM coolant temperature sensor short circuit after driving through water

✓
Owner reported the air conditioning suddenly stopped heating after driving in heavy rain, with a yellow motor overheating warning on the dash. Diagnosis revealed active fault code B122B11. Inspection found obvious water stains inside the front compartment PTC low-voltage connector. The pins had oxidised and blackened, causing a short circuit between the signal pin and earth pin. Cause: Connector seal deterioration allowed water ingress during wading. Repair: Cleaned the connector pins using electronic cleaner, replaced the waterproof seal, applied an appropriate amount of lubricating grease around the connector to improve sealing, and replaced the PTC temperature sensor (corroded). Recommend checking the sealing condition of other low-voltage connectors in the front compartment.
BYD DTC AI AnalysisFrom Chinese market (translated)

Improperly secured sensor wiring harness following repair caused a short circuit in Qin EV

✓
The vehicle had previously had the PTC heater replaced at a third-party workshop. One week later, fault code B122B11 appeared. Inspection found the repair had not properly secured the sensor wiring harness. The harness was hanging loose and had contacted the PTC metal housing. The high temperature melted the insulation, causing it to stick to the housing and shorting the signal wire to the housing. Repair action: Replaced the damaged harness section (from the connector to the main harness), protected it with high-temperature corrugated tubing (125°C rated), routed the harness to maintain at least 20mm clearance from the PTC housing, and secured it with special high-temperature cable ties at the factory mounting points.
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin Pro DM — Sensor internal short circuit triggered PTC protection

✓
The air conditioning heating stopped automatically two minutes after starting the vehicle. No warning appeared on the instrument panel, but DTC B122B11 occurred intermittently. Resistance measured 0.8Ω at room temperature (normal is 2.5kΩ), indicating the internal thermistor had short-circuited. Replaced the PTC outlet water temperature sensor (part number likely 6A-812XXXX series; confirm against VIN) to resolve the fault. The sensor operated long-term in a high-temperature, high-pressure environment (PTC outlet water temperature can reach 80°C, pressure 1.2 bar); internal seal failure caused the component to short.
BYD DTC AI AnalysisFrom Chinese market (translated)

Multiple sensor short circuits cause thermal management failure in a Qin EV300.

✓
Extreme case: Vehicle simultaneously logged B122B11, B122A13 (coolant temperature sensor circuit open) and multiple other temperature sensor faults. The rubber grommet of the front wiring harness main trunk at the firewall penetration had fallen off, allowing the harness to chafe against the sharp sheet metal edge and damage multiple wires. The coolant temperature sensor signal wire shorted to ground, and other sensor power supply wires had intermittent opens. Repair: Repaired the damaged wires, reconnected them with waterproof splice connectors, repaired the firewall rubber grommet (filled with sealant) and reinstalled it. Routed the harness smoothly without tension.
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. Sources: [1]