AU
B121013

This DTC indicates an open circuit in the left heat sink temperature sensor of the thermal management system PTC (Positive Temperature Coefficient) heater — Atto 3

Thermal Management System

This DTC indicates an open circuit in the left heat sink temperature sensor of the thermal management system PTC (Positive Temperature Coefficient) heater.

This sensor, typically an NTC (Negative Temperature Coefficient) thermistor, monitors the real-time surface temperature of the PTC ceramic heating element to prevent overheating.

The ECU determines an open circuit when it detects a continuously high sensor signal voltage (typically the 5V reference voltage returning without voltage division).

This fault triggers a protective shutdown of the PTC heater, resulting in no warm air in the cabin.

In extreme cases, if the PTC overheats and the failed sensor cannot provide feedback, it may trigger a high-voltage safety cut-off or risk thermal runaway.

3
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1Internal open circuit in the temperature sensor body: The NTC thermistor fractures internally due to aging, thermal shock, or mechanical stress, resulting in infinite resistance.
  • 2Wiring harness connector fault: Prolonged high temperatures cause oxidized or backed-out pins in the connector near the PTC assembly, or a loose retaining clip results in poor contact.
  • 3Wiring harness physical damage: Vehicle vibration causes the wiring harness in the motor compartment or left side of the front compartment to rub against sharp edges, breaking the signal wire or ground wire.
  • 4PTC control board sampling circuit fault: A damaged sampling resistor, damaged filter capacitor, or cold solder joint on the control board causes the ECU to incorrectly detect a sensor open circuit.
  • 5Improper sensor installation: Sensor not seated correctly during maintenance, or excessive torque cracked the housing and broke the internal leads.
What To Do
  • 1
    Use VDS to read the complete fault information and freeze frame data, confirm the PTC operating status and ambient temperature when the fault occurred, and check for other thermal management-related fault codes.
  • 2
    Visually inspect the PTC heater left temperature sensor. Check the wiring harness sleeve for damage and the connector for looseness, water ingress, or burn marks.
  • 3
    Disconnect the sensor connector. Measure the resistance between the two sensor terminals using a multimeter. At room temperature (25°C), the NTC resistance should be 2kΩ-10kΩ (refer to the vehicle repair manual for specific values). If the reading shows OL (open circuit), replace the sensor.
  • 4
    Check wiring harness continuity: Measure the continuity of the signal and ground wires between the sensor connector and the air conditioning controller (or PTC controller). Resistance must be less than 1Ω. Check for short to ground and short to power.
  • 5
    Check the reference voltage: Turn the ignition ON. Without disconnecting the connector, measure the voltage between the signal wire and ground. The normal reference voltage is 5V. If the voltage is 5V but the sensor side has an open circuit, verify normal sampling at the ECU.
  • 6
    Repair or replace: If the sensor is damaged, replace with an OEM temperature sensor (apply thermal grease and tighten to the specified torque); if the wiring harness is faulty, repair or replace the harness and verify the waterproof seal.
  • 7
    Clear the fault code and perform a PTC function test: set the heater to the maximum temperature, verify the left heat sink temperature in the data stream is normal (it should rise gradually as the PTC operates), and confirm the fault code does not return.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin Pro DM: No warm air in winter with PTC protective shutdown

✓
The vehicle came in with no warm air from the heater and an intermittent coolant temperature warning light. The scan tool retrieved DTC B121013, and freeze frame data showed the PTC heater was running at high power when the fault occurred. Inspection found the left radiator temperature sensor wiring harness rubbing against the body panel on the left side of the front compartment. Long-term vibration had worn through nearly all copper strands, leaving only a few intact. The circuit conducted intermittently when cold but opened completely after thermal expansion when hot. Repaired the harness, rerouted and secured it away from sharp edges. Fault resolved.
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Qin EV300 PTC not working after accident repair

✓
After front-end collision repairs, the PTC heater wouldn't turn on when the heater was switched on. Retrieved DTC B121013. Inspection found the engine bay wiring harness had been replaced during the repair, but the left-side heat sink temperature sensor connector pin wasn't fully seated—one pin had backed out, causing poor contact. Re-seated the pin, locked the connector catch into place, cleared the DTC, and the PTC worked normally.
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin Pro DM: Internal open circuit in sensor causes intermittent fault

✓
Customer reported intermittent heater operation. DTC B121013 stored as a history code. Wiring harness continuity normal, connectors free of faults. Removed the left side heatsink temperature sensor and measured resistance. Resistance unstable at ambient temperature; tapping the sensor housing caused the reading to jump to infinity. Determined the internal NTC element had a cold solder joint or fracture. Replaced the PTC temperature sensor and monitored for one week. Fault has not reoccurred.
Other Atto 3 Fault Codes
P0A0DBattery Energy Control Module Requested MIL IlluminationP0AA6Hybrid Battery Voltage System Isolation FaultP1A00Drive Motor "A" Performance — BYD proprietaryB1108For 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.
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]