AU
B121111

This DTC indicates a short to ground in the PTC (Positive Temperature Coefficient) heater left heat sink temperature sensor signal circuit, or an internal sensor short circuit — Atto 8

Thermal Management System

This DTC indicates a short to ground in the PTC (Positive Temperature Coefficient) heater left heat sink temperature sensor signal circuit, or an internal sensor short circuit.

BYD Qin series vehicles typically integrate this sensor into the PTC heater assembly to monitor heater core temperature and prevent overheating.

During a short circuit, the air conditioning controller detects an abnormally low voltage signal (near 0V) and triggers protection logic to cut PTC power, disabling the heating function.

Because the PTC heater draws high operating current (up to tens of amperes), temperature monitoring failure risks overheating or fire.

Therefore, the system classifies this as a severe fault (Level 3).

4
Cases Logged
5
Causes
Likely Causes — Atto 8
  • 1PTC temperature sensor wiring harness insulation damaged: High temperatures and vibration in the motor compartment cause the sensor wiring harness to rub against metal body edges, shorting the signal wire to ground.
  • 2Internal short circuit in sensor body: NTC thermistor package failure or moisture ingress causes resistance to drop abnormally to nearly 0 Ω.
  • 3PTC heater assembly internal fault: Insulation layer breakdown between the heat sink temperature sensor and the PTC ceramic element causes a short circuit.
  • 4Air conditioning controller internal short circuit: The pull-up or pull-down resistor in the controller's internal sampling circuit or the ADC interface shorts to ground, triggering a false sensor fault.
  • 5Connector water ingress and corrosion: After driving through water or washing the vehicle, poor PTC connector sealing causes a short circuit between pins or a short to ground.
What To Do
  • 1
    Connect the VDS2000 diagnostic tool. Read the complete fault codes and freeze frame data. Confirm the fault frequency (current/history). Check for an accompanying B121013 (open circuit) fault appearing alternately, indicating an intermittent contact issue.
  • 2
    Disconnect the battery negative terminal, wait 5 minutes, then unplug the temperature sensor connector on the PTC heater (usually located near the heater core, 2-pin black plug). Measure the resistance at the sensor terminals: a normal NTC sensor measures 10 kΩ ±5% at 25°C. A reading of 0–10 Ω indicates an internal short circuit in the sensor.
  • 3
    If sensor resistance is normal, measure resistance to ground at the harness-side signal pin. Normal resistance is infinite. If continuity exists, inspect the wiring harness routing. Focus on insulation damage at the firewall pass-through and harness retaining clip edges.
  • 4
    Check the PTC heater assembly: If the wiring harness is normal, remove the PTC assembly and measure the insulation resistance between the sensor connector and the housing. The resistance must be greater than 20 MΩ. Otherwise, the PTC assembly has an internal short circuit. Replace the PTC heater assembly (part number may be BC-8123100, etc.).
  • 5
    Check the control module: If all above checks are normal, disconnect the air conditioning controller (or thermal management integrated module) and measure the harness-side resistance to ground. After ruling out a wiring harness fault, check the controller internal circuit. Flash the latest software version or replace the controller if necessary.
  • 6
    Post-repair procedure: After replacing the faulty component, clear the fault code. Start the vehicle and set the heater to maximum temperature. Observe the 'PTC left-side heat sink temperature' value in the data stream; it should rise during heating (normal range: -40°C to 150°C). Verify the fault code does not return.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin Pro DM: PTC wiring harness wear causes intermittent short circuit

✓
A 2019 Qin Pro DM had an intermittent 'motor overheating' warning light and intermittent heater failure. The diagnostic tool recorded current fault B121111; freeze frame data showed an ambient temperature of -5°C, suggesting heavy heater use in winter was the cause. Lifted the vehicle and found the PTC sensor wiring harness had chafed against a sharp metal edge where it passed through the firewall, wearing through the insulation and exposing the copper wire. Repaired the harness and fitted abrasion-resistant sleeving. Fault resolved.
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin EV300 PTC assembly internal sensor short circuit

✓
2017 BYD Qin EV300: no heat from the heater. Read DTC B121111. Measured PTC sensor resistance at only 0.8 Ω. Disconnected the sensor — module still reported short circuit, ruling out a wiring harness fault. Removed and inspected the PTC heater. Found obvious burn marks on the temperature sensor probe; the insulation layer between the ceramic plate and metal heat sink had broken down. Replaced PTC heater assembly (including sensor). Heater now works normally, DTC cleared.
BYD DTC AI AnalysisFrom Chinese market (translated)

Sensor connector shorted after water ingress.

✓
After driving through water, the air conditioning warning light illuminated. Read DTC B121111. Found water inside the PTC sensor connector (located near the right front wheel arch liner), with verdigris corrosion between the pins causing a short circuit. Cleaned the connector, applied conductive grease and replaced the waterproof seal. Fault resolved. Recommend checking the water wading depth sensor and drain holes for blockages.
BYD DTC AI AnalysisFrom Chinese market (translated)

AC controller software falsely reporting fault

✓
2018 BYD Qin Pro DM. Replaced the PTC assembly, but DTC B121111 remained. Measured the new sensor resistance: 9.8kΩ (normal). Wiring harness showed no shorts. Suspected a control module fault. Checked the AC controller software version—it was V1.2, which has a known temperature sampling circuit bug. Updated the controller software to V1.5, cleared the DTC, and it did not recur.
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]