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B121809

This DTC indicates a functional failure of the IGBT (Insulated Gate Bipolar Transistor) power module in the thermal management system, specifically the electric compressor or PTC heater controller — Seal U

Thermal Management System

This DTC indicates a functional failure of the IGBT (Insulated Gate Bipolar Transistor) power module in the thermal management system, specifically the electric compressor or PTC heater controller. 'Uncontrolled, stuck on or stuck off' indicates the IGBT has lost gate control capability.

A stuck-on state means the IGBT conducts continuously, creating a high-voltage short circuit risk or forcing the component to run continuously at full power.

A stuck-off state means the IGBT cuts off completely, resulting in a loss of function.

Typical causes include gate drive circuit damage (e.g., isolation driver chip or gate resistor faults), IGBT breakdown or open circuit, abnormal drive power supply (±15V supply failure), or abnormal PWM signal output from the control board MCU.

In high-voltage environments, this fault can trigger overcurrent protection lockout, cause insulation faults, or induce thermal breakdown of the power module, posing a severe safety risk.

5
Cases Logged
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin Pro DM: No heating in winter, PTC controller IGBT breakdown

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No warm air from the heater in winter; the instrument cluster displayed “Thermal Management System Fault”. Read fault code B121809 (current fault). Removed and inspected the PTC controller and found obvious burn-through holes on the IGBT module surface. C-E resistance measured near 0 Ω (short circuit breakdown), and the gate drive resistor (10 Ω) had burnt black and gone open circuit. Analysis showed the PTC had been operating at full power for extended periods; poor heat dissipation caused IGBT junction temperature to exceed 150 °C, leading to thermal breakdown. Replaced the PTC controller assembly (integrated IGBT and driver board) and cleaned scale from the PTC water jacket to improve heat dissipation. Fault resolved.
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin EV300 electric compressor made abnormal noise then shut down; IGBT stuck-open fault

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The customer reported that when turning on the AC, the compressor emitted a buzzing noise, then stopped working and would not restart. The diagnostic scan tool displayed codes B121809 and B121A09 (#1 IGBT driver chip fault). Removed the compressor controller and measured a 0.3 V voltage drop between the collector and emitter of the V-phase upper bridge arm IGBT (which should normally be in cutoff), determining the IGBT was permanently on (uncontrolled conduction). Disassembled the compressor and found the internal motor winding insulation was normal, confirming the controller IGBT itself had failed. Since the BYD electric compressor has an integrated IGBT and controller that does not support individual power module replacement, replaced the entire electric compressor assembly, also replaced the desiccant, evacuated and recharged the refrigerant, and restored normal operation.
BYD DTC AI AnalysisFrom Chinese market (translated)

Cold solder joint on the drive power supply causes intermittent B121809 fault.

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Air conditioning intermittently cut out while driving but returned to normal after a restart. Fault code: intermittent B121809. Lightly tapping the PTC controller housing during inspection easily reproduced the fault. Disassembly revealed dry solder joints on the pins of the isolated power supply module on the driver board (which provides +15V to the IGBT drive). Vehicle vibration or temperature changes caused poor contact, dropping the IGBT drive voltage below 8V so it could not conduct properly. Resoldered the power module pins using a hot air gun and applied thermal paste to improve heat dissipation. The fault has not recurred.
BYD DTC AI AnalysisFrom Chinese market (translated)

B121809 reported after wading through water, accompanied by insulation fault

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After the vehicle waded through water, the instrument cluster displayed multiple high-voltage faults. Retrieved DTC B121809 and a low insulation resistance fault. Inspection found the PTC controller mounted low on the chassis; the housing seal ring had aged, allowing water ingress. Disassembly revealed electrolytic corrosion on the internal IGBT gate driver circuit board. A leakage path formed between the gate and emitter, causing the IGBT to operate uncontrollably. Due to severe circuit board corrosion, replaced the PTC controller assembly. Also checked the body floor seal integrity, replaced the seal strips, and dried the entire high-voltage system. Insulation returned to normal.
BYD DTC AI AnalysisFrom Chinese market (translated)

Replaced PTC, falsely reported B121809; software version mismatch

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After replacing the PTC heater with a new unit, DTC B121809 triggered each time the heater was turned on, even though the PTC worked normally (hot air). Comparing the old and new hardware versions showed the new part had a newer revision (V03), while the VCU software was still the older 2018 version. The mismatch between the new PTC controller's IGBT self-test threshold and the old software caused a false DTC. After upgrading the VCU software to the latest 2019 version, the fault code disappeared and the system worked normally. Technicians should verify software and hardware compatibility when replacing high-voltage components.
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: Community This information is for reference only. Always consult a qualified technician for diagnosis and repair. Do not attempt high-voltage system repairs yourself.