AU
B121C09

This fault code indicates a functional failure of the No — Seal U

Thermal Management System

This fault code indicates a functional failure of the No. 3 IGBT (Insulated Gate Bipolar Transistor) driver chip in the thermal management system.

In BYD Qin series new energy vehicles, the A/C PTC (Positive Temperature Coefficient) heater uses an IGBT module for high-power PWM modulation control, where "3#" refers to the third-phase drive channel or the third IGBT unit.

The IGBT driver chip converts low-voltage logic signals from the controller into high-voltage, high-frequency pulse signals to drive the IGBT gate, precisely regulating PTC heater power (0-100% duty cycle control).

This fault occurs when the driver chip fails to output the normal gate drive voltage (typically a +15V/-8V push-pull signal), or when the chip’s internal desaturation protection or overcurrent protection triggers abnormally, preventing the IGBT from conducting or forcing it to remain off.

This directly causes partial or complete PTC heater failure, affecting cabin heating.

In low-temperature environments, it also disables battery pack preheating, potentially triggering a vehicle thermal management fault and limiting power output.

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Cases Logged
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Causes
Likely Causes — Seal U
  • 1The internal IGBT driver chip of the PTC heater assembly burned out due to overheating, or gate oxidation reduced its drive capability.
  • 2Open circuit or resistance drift in the gate resistor (typically 10-47Ω) of the IGBT gate drive circuit, attenuating the drive signal.
  • 3PTC heating element insulation resistance drops due to aging or moisture ingress, increasing IGBT drain-source leakage current and triggering driver chip protection.
  • 4Poor contact, backed-out terminals, or oxidized pins at the low-voltage drive wiring harness connector (such as GJ106 or GJK3) between the air conditioning controller and the PTC, causing intermittent drive signal interruption.
  • 5Poor circulation in the thermal management system cooling circuit or dried thermal grease causes the IGBT module junction temperature to exceed 150°C, resulting in thermal damage.
What To Do
  • 1
    High-voltage safety procedure: Wear insulated gloves, disconnect the front compartment manual service disconnect (MSD), wait at least 5 minutes to ensure the high-voltage capacitors fully discharge, and use a multimeter to confirm the high-voltage bus voltage is <60V.
  • 2
    Fault code analysis: Connect the VDS2000/VDS diagnostic tool and read the B121C09 freeze frame data. Record the PTC operating duty cycle, IGBT temperature, and high-voltage bus voltage at the time of the fault to confirm if it is a current hard fault.
  • 3
    Low-voltage wiring harness inspection: Check the low-voltage drive wiring harness from the air conditioning controller to the PTC heater (including the PWM signal wire, fault feedback wire, and 15V power wire). Specifically check pins 2 and 3 of connector GJ106 for oxidation, terminal spread, or looseness.
  • 4
    High-voltage insulation test: Use a 1000V megohmmeter to measure the insulation resistance between the PTC high-voltage input terminal and body ground. Standard value: >500MΩ. A low insulation value (<20MΩ) indicates a breakdown of the PTC internal heating element or IGBT module.
  • 5
    Drive circuit measurement: Reconnect the low-voltage battery. Do not disconnect the PTC connector. Use an oscilloscope to measure the input PWM waveform (normally a 5V square wave) and output gate waveform (normally a 15V square wave) of the No. 3 IGBT driver chip. If the input is normal but the output is abnormal, the driver chip is faulty.
  • 6
    Component-level repair: Epoxy resin potting typically encapsulates the IGBT driver chip inside the PTC heater, preventing separate replacement. Replace the complete PTC heater assembly (verify the part number by vehicle model, e.g., Qin EV300: LBC-8113100).
  • 7
    System calibration and verification: After replacement, clear the fault codes and perform the 'Air Conditioning System Self-Learning' or 'PTC Calibration' procedure (if available in VDS). Set the heater to HI, read the data stream to confirm the 3# IGBT drive duty cycle is normal (varies with temperature setting), and observe the air outlet temperature rise above 40°C within 3 minutes.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin Pro DM PTC heater IGBT driver chip overheated and burnt out

✓
The 2018 Qin Pro DM developed intermittent cabin heating in cold conditions (-20°C), eventually losing all heat output, with the dashboard displaying 'Thermal Management System Fault'. The scan tool retrieved DTCs B121C09 and B121B09 (No. 2 IGBT fault). Removed and inspected the PTC heater assembly. Found burn marks on the surface of the No. 3 IGBT driver chip, with the gate resistor (22Ω) open. Root cause: The owner regularly used DC fast charging while running the heater on MAX, causing the PTC to operate at full load for extended periods with insufficient cooling. Replaced the PTC assembly with an improved unit featuring a larger heat sink, and advised the owner to avoid prolonged high-power heater use during charging. This resolved the fault.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

Poor contact in the Qin EV300 PTC drive wiring harness caused IGBT failure.

✓
2017 Qin EV300 developed intermittent heater failure after driving on rough roads. The diagnostic tool showed B121C09 (intermittent fault). Static measurements showed normal PTC insulation and resistance. Checked the PTC low-voltage connector GJK3 above the passenger footwell and found pin 3 IGBT drive signal (usually yellow wire) had backed out 0.5mm, causing poor contact while driving. The drive chip received abnormal pulses and reported function failure. Used a dedicated pin removal tool to repair the pin retention clip, re-crimped and applied conductive paste (DJK-03). Cleared the fault codes and monitored for 3 months with no recurrence.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

Drive mismatch fault after aftermarket PTC replacement on Qin Pro DM

✓
After fitting an aftermarket PTC heater during accident repairs, the vehicle immediately set DTC B121C09 on power-up, and the code would not clear. Compared the original and aftermarket parts: the aftermarket unit used an early single-channel IGBT driver IC, while the vehicle’s controller software (post-2019 version) uses three-phase differential drive signals. This prevented channel 3 from recognising the drive waveform. Ordered an OEM PTC assembly matched to the VIN (part number must match factory configuration), reflashed the ACU to the latest software, and resolved the fault.
Original source ↗
BYD DTC AI AnalysisFrom Chinese market (translated)

Qin EV300 air conditioning controller drive power fault triggered cascade failures

✓
The vehicle failed to enter the Ready state. Multiple high-voltage system DTCs accompanied B121C09. Routine PTC inspection found no abnormalities. In-depth measurement revealed that the air conditioning controller's 15V IGBT drive power supply (feeding three IGBT drive chips) had excessive ripple and a voltage of only 11.2V, below the chip's minimum operating voltage (12V). Disassembly of the air conditioning controller revealed a bulging filter capacitor (1000μF/25V) on the internal switch-mode power supply board. After replacing the capacitor, the 15V supply returned to normal, all DTCs cleared, and the vehicle returned to normal operation. This case demonstrates that drive chip faults may originate from the upstream power supply rather than the PTC itself.
Original source ↗
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]