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 — Seal U

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 — Seal U
  • 1Temperature sensor internal open circuit: Aging, thermal shock, or mechanical stress causes the NTC thermistor to fracture internally, resulting in infinite resistance.
  • 2Wiring harness connector fault: Prolonged high-temperature exposure causes terminal oxidation or push-out in the connector near the PTC assembly, or a loose locking clip causes poor contact.
  • 3Physical damage to the wiring harness: Vehicle vibration causes the wiring harness in the engine compartment or left side of the front compartment to rub against sharp edges, breaking the signal 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: Incorrectly seating the sensor during repair, or applying excessive torque causing housing cracks and broken 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 accompanying 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 and use a multimeter to measure the resistance between the two sensor terminals. At normal temperature (25°C), the NTC resistance should be 2 kΩ–10 kΩ (refer to the vehicle repair manual for specific values). If the multimeter displays OL (open circuit), replace the sensor.
  • 4
    Check wiring harness continuity: Measure continuity of the signal wire and ground wire 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. Measure the signal wire voltage to ground without disconnecting the connector. The reference voltage should be 5V. If the voltage is 5V but the sensor side has an open circuit, confirm ECU-side sampling is normal.
  • 6
    Repair or replace: If the sensor is damaged, replace it 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, observe the left heat sink temperature in the data stream to verify normal operation (the temperature must rise gradually during PTC operation), 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 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. Sources: [1]