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B122A13

DTC B122A13 indicates an open circuit fault in the thermal management system coolant temperature sensor circuit — Atto 3

Thermal Management System

DTC B122A13 indicates an open circuit fault in the thermal management system coolant temperature sensor circuit.

In models such as the BYD Qin EV300, this sensor typically mounts in the PTC (Positive Temperature Coefficient) water heater or the battery thermal management circuit.

It monitors coolant temperature to control heating power and battery thermal management strategies.

An open circuit means the ECU detects a continuously high sensor signal voltage (typically near the 5V reference voltage) and cannot acquire the actual temperature signal.

This fault forces the PTC heater into protection mode and halts operation, resulting in no warm air in the cabin.

It may also affect the battery preheating function, restricting charging speed or preventing charging in low-temperature environments.

In severe cases, the vehicle may trigger a thermal management system fault and enter limp mode.

5
Cases Logged
5
Causes
Likely Causes — Atto 3
  • 1The coolant temperature sensor internal thermistor is open or damaged, preventing a complete circuit.
  • 2Sensor wiring harness connector is loose, has a backed-out terminal, or makes poor contact, which is particularly prone to occur in the high-temperature, high-vibration environment of the engine compartment.
  • 3Wiring harness worn or broken, commonly at high-friction points such as the firewall pass-through and harness retaining bracket edges.
  • 4PTC heater assembly internal temperature sensor fault (on some models, the sensor is integrated into the PTC assembly and is not replaceable separately).
  • 5Internal signal acquisition circuit fault in the thermal management controller (air conditioning controller) causing a false open circuit detection.
What To Do
  • 1
    Connect the dedicated BYD diagnostic tool (VDS or ED400), read the DTC freeze frame and data stream, identify which coolant temperature sensor has no signal output, and record the ambient temperature at the time of the fault.
  • 2
    Visually inspect the sensor wiring harness connector for looseness, water ingress, or corrosion. Focus on connectors in the high-temperature area near the PTC heater. Clean and reconnect if necessary.
  • 3
    Disconnect the sensor connector and measure the sensor resistance with a multimeter. Compare the reading against the temperature-resistance characteristic curve in the repair manual (typically about 2-10kΩ at 25°C) to verify the sensor is normal.
  • 4
    Turn the ignition switch to the ON position. Measure the reference voltage (usually 5V) and check the ground wire at the harness-side connector to confirm the ECU power supply is normal.
  • 5
    Measure wiring harness continuity between the sensor connector and the thermal management controller (or air conditioning controller), focusing on harness integrity at the inline connector and firewall pass-through.
  • 6
    If the sensor is integrated into the PTC heater assembly and confirmed faulty, replace the PTC heater assembly. If the sensor is separate, replace the sensor.
  • 7
    After completing the repair, clear the fault code, perform a PTC function test and road test, and verify the temperature reading in the data stream is normal and the fault code does not return.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

BYD Qin EV300 PTC heater inlet sensor open circuit causing no heating

✓
Vehicle came in with no heating and the thermal management system warning light on. Retrieved DTC B122A13. Live data showed PTC inlet temperature at -40°C (out of range low). Measured infinite resistance at the PTC heater inlet temperature sensor, confirming an internal open circuit. Since the sensor is integrated in the PTC water heater assembly, replaced the assembly. Fault cleared, heating returned to normal.
BYD DTC AI AnalysisFrom Chinese market (translated)

Wiring harness chafed through at the firewall penetration, causing an intermittent open circuit.

✓
The vehicle intermittently set DTC B122A13 on rough roads but operated normally on smooth surfaces. During a shake test, we found the firewall wiring harness clip had worked loose, allowing the harness to chafe against the metal panel edge. This wore through the insulation and partially fractured the copper cores, creating an intermittent high-resistance connection. We repaired the harness, rerouted the wiring, fitted a protective sleeve, and secured the clip. The fault has not returned.
BYD DTC AI AnalysisFrom Chinese market (translated)

Water ingress corroded the connector, causing poor contact.

✓
Drove after rain and developed a fault. Scan tool read DTC B122A13. Found the coolant temperature sensor connector in the engine bay had water residue inside. Oxidised and blackened pins caused high contact resistance, leading the ECU to misdiagnose an open circuit. Cleaned the connector with electrical contact cleaner, sanded the pins with fine sandpaper, applied conductive paste, and sealed the connection against water. Fault resolved.
BYD DTC AI AnalysisFrom Chinese market (translated)

Open circuit in battery cooling circuit temperature sensor limits charging power.

✓
During fast charging, power was only 20kW (normal 60kW) and the instrument cluster displayed a charging system fault. Read multiple thermal management DTCs, including B122A13 (battery coolant temperature sensor). Measured the battery pack coolant inlet temperature sensor and found an open circuit. This prevented the BMS from accurately determining battery temperature, so it limited charging current. Replaced the battery coolant temperature sensor and charging power returned to normal.
BYD DTC AI AnalysisFrom Chinese market (translated)

False positive due to internal circuit fault in AC controller

✓
Replaced the coolant temperature sensor, but DTC B122A13 persisted. Checked wiring harness continuity and power supply—both normal. Measured voltage from sensor signal wire to ground: 5V (normal should be a varying voltage). Suspected an internal signal acquisition circuit fault in the HVAC controller (thermal management controller). Replaced the HVAC controller and programmed the new unit. The DTC cleared and live data returned to normal.
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]