AU
B122111

DTC B122111 indicates a short to ground or abnormally low signal circuit impedance in the air conditioning/thermal management system RHS Fin Temperature Sensor — Seal U

Thermal Management System

DTC B122111 indicates a short to ground or abnormally low signal circuit impedance in the air conditioning/thermal management system RHS Fin Temperature Sensor.

This sensor is typically an NTC (negative temperature coefficient) thermistor with a normal operating voltage range of 0.5-4.5V.

The ECU triggers this fault when the signal voltage remains below 0.1V (short-circuit threshold) for longer than a set time (typically 2-5 seconds).

In Qin EV/Qin Pro DM models, this sensor mounts on the right-hand fin of the PTC heater assembly or at the heater core outlet to monitor the heater operating temperature and prevent overheating.

A short circuit prevents the thermal management ECU from obtaining accurate temperature data.

The ECU then triggers a protection mechanism that shuts down the PTC heater and restricts the air conditioning heating function.

In extreme cases, this condition opens the high-voltage interlock and forces the vehicle into limp mode.

4
Cases Logged
5
Causes
Likely Causes — Seal U
  • 1Sensor body internal short circuit: Long-term high-temperature aging or voltage breakdown causes an internal short circuit in the NTC thermistor; the resistance value approaches 0 Ω.
  • 2Wiring harness short to ground: High temperatures in the engine compartment degrade and damage the sensor wiring harness insulation. The exposed wiring contacts the metal body frame or lines, creating a short to ground.
  • 3Connector water ingress short circuit: Poor sealing of the sensor connector (usually located on the right side of the firewall or on top of the PTC heater) allows water entry when driving through water or washing the motor compartment, causing a short circuit between pins or between a pin and the housing.
  • 4PTC heater assembly fault: Internal heat sink fins physically contact the temperature sensor mounting seat, or an internal circuit fault shorts the signal wire to the heater housing (ground).
  • 5ECU sampling circuit fault: Component breakdown in the A/D converter front-end circuit inside the air conditioning controller or thermal management module shorts the signal input terminal to ground (relatively rare).
What To Do
  • 1
    Diagnostic confirmation: Use VDS or a dedicated diagnostic tool to read the complete DTC. Check the voltage value in the freeze frame data (should be close to 0V). Confirm whether it is a current fault (Current DTC) rather than a historical fault.
  • 2
    Physical inspection: Disconnect the 12V battery negative terminal and wait 5 minutes for the high-voltage system to power down. Inspect the PTC heater assembly and the right heat sink temperature sensor connector located on the right side of the front compartment for water ingress, corrosion, deformed pins, or wiring harness damage.
  • 3
    Circuit measurement: Disconnect the sensor connector. Use a multimeter to measure the resistance between the two terminals on the sensor side (normal: 1-10kΩ, varies with temperature). Measure the resistance between the signal wire on the harness side and ground (normal: >1MΩ; continuity indicates a short to ground).
  • 4
    Signal voltage check: Reconnect connector, turn ignition switch ON (do not start), and measure signal wire voltage (normal: 0.5-4.5V; close to 0V if shorted). If voltage is normal but fault code remains, check for intermittent short circuit or ECU fault.
  • 5
    Component replacement verification: If sensor resistance is abnormal (short circuit), replace the right heat sink temperature sensor (part number usually included with the PTC heater); if the wiring harness has a short circuit, repair the damaged insulation or replace the wiring harness; if the sensor and wiring harness are normal, replace the air conditioning controller/thermal management module.
  • 6
    System verification: Clear the fault code, start the vehicle, and set the heater to maximum. Observe the data stream to verify the right-side heat sink temperature rises normally during PTC operation (typically 30-80°C). Confirm the fault does not recur.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin Pro DM heater failure after wading

✓
After driving through approximately 30 cm of water, the instrument cluster displayed a high coolant temperature warning and the heater produced no warm air. VDS retrieved active DTC B122111; freeze frame data showed sensor voltage 0.02V. Inspection of the front compartment found water inside the right-side PTC heater connector and oxidised pins. Procedure: Cleared the water from the connector, cleaned with WD-40 Precision Electrical Contact Cleaner, blow-dried, then applied conductive grease. Insulation resistance returned to ∞. Fault resolved, heater function restored.
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin EV300 PTC heater internal short circuit

✓
The customer reported vehicle judder and a powertrain fault warning when turning on the heater in winter. The scan tool displayed DTC B122111 and multiple high-voltage interlock-related faults. Inspected the right-hand radiator temperature sensor harness – no visible damage. Disconnected the sensor and measured its resistance: 0.3 Ω (should be 2–3 kΩ at 25°C). This confirmed internal sensor breakdown. Replaced the PTC heater assembly (sensor not available separately). Fault resolved. Root cause: prolonged high-temperature operation of the heater caused the thermistor to age and short-circuit.
BYD DTC AI AnalysisFrom Chinese market (translated)

Wiring harness chafing caused an intermittent short circuit.

✓
While driving, the heater intermittently stopped working. DTC B122111 was stored as a history code. Inspection found no obvious short circuits and the sensor resistance measured normal. A closer look revealed the right-side heat sink temperature sensor harness had chafed against the metal edge where it passes through the firewall, wearing through the insulation and exposing copper wire. Vehicle vibration caused intermittent shorts to ground. Repair: wrapped the damaged area with electrical tape, rerouted the harness and added protective sleeving, then secured the harness to prevent contact with the body. A one-week road test showed no recurrence.
BYD DTC AI AnalysisFrom Chinese market (translated)

False positive fault - ECU software issue

✓
The 2018 Qin Pro DM had DTC B122111 from new. Sensor resistance and voltage measured normally, with no short circuit in the wiring. A service bulletin revealed a software calibration defect in the HVAC controller on some vehicles from this model year, causing false short-circuit detection at ambient temperatures below -10°C. Solution: Update the HVAC controller software to version V2.1 or later. After the update, the fault code disappeared and the heating function worked normally in low temperatures. This was a software false alarm—no hardware replacement needed.
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]