AU
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 — Atto 3

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
5
Causes
Likely Causes — Atto 3
  • 1Overheating damage to the internal IGBT module of the PTC heater controller or electric compressor controller (due to insufficient coolant, dried thermal grease, or heat build-up from prolonged high-current operation)
  • 2IGBT gate drive circuit fault, including damaged isolated driver optocoupler (such as ACPL-330J), burnt open gate resistor (typically 10-22Ω), or gate Zener diode breakdown.
  • 3Drive power module fault: The DC-DC converter on the control board cannot provide a stable +15V/-8V drive voltage, preventing the IGBT from switching on or off properly.
  • 4Physical damage to the IGBT module, including chip bond wire lift-off, silicon die thermal runaway, or voltage surge-induced collector-emitter breakdown.
  • 5Controller software or hardware compatibility issues, incorrect PWM dead-time setting causing upper and lower bridge arm shoot-through, or EMC interference causing abnormal drive signals.
What To Do
  • 1
    Safety preparation: Wear insulated gloves, disconnect the high-voltage Manual Service Disconnect (MSD), wait at least 5 minutes for the high-voltage capacitors to fully discharge, and use a multimeter to confirm the high-voltage bus voltage is <60V.
  • 2
    Fault confirmation: Use a VDS2000 or X-431 diagnostic tool to read complete fault codes and freeze frame data. Record the IGBT temperature, high-voltage bus voltage, phase current, and PWM duty cycle at the time of the fault. Confirm whether the fault is current (present) or historical (history).
  • 3
    Insulation test: Use a 1000V megohmmeter to measure the insulation resistance of the air conditioning high-voltage components (compressor, PTC) to the vehicle body. Standard: >550MΩ. If insulation is low, further isolate the fault to an IGBT breakdown or leakage in other high-voltage components.
  • 4
    Controller removal and inspection: Remove the electric compressor controller or PTC controller. Visually inspect the IGBT module surface for burn marks, cracks, or carbonization. Smell for a burnt odor and inspect the electrolytic capacitors on the driver board for bulging.
  • 5
    Static measurement: Use a multimeter on the diode setting to measure the forward and reverse voltage drops across the C-E terminals and the G-E terminals on the upper and lower bridge arms of each IGBT module phase. Normally, the G-E terminals show a voltage drop of approximately 0.6 V (built-in diode), and the C-E terminals remain non-conducting. A reading of 0 V or a short circuit indicates IGBT breakdown; an infinite reading indicates a possible open circuit.
  • 6
    Dynamic test: Connect the low-voltage power supply (12V) and use an oscilloscope to monitor the gate drive signal. Verify a normal +15V pulse during the turn-on command, and -8V or 0V during turn-off. If the drive signal is normal but the IGBT does not operate, replace the IGBT module.
  • 7
    Component replacement: Upon confirming a faulty IGBT, replace the power module with the same model (such as Infineon HybridPACK or BYD in-house module). Simultaneously replace damaged gate resistors and driver optocouplers, reapply thermal grease (thermal conductivity ≥3 W/m·K), and tighten to the standard torque (usually 3-5 N·m).
  • 8
    System reset: After reassembly, fill with coolant and bleed the system. Clear the fault code and perform an air conditioning system function test. Observe the compressor or PTC current and temperature curves to verify normal operation. Monitor continuously for 30 minutes to confirm no recurrence.
Real-World Cases
BYD DTC AI AnalysisFrom Chinese market (translated)

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

✓
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

✓
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.

✓
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

✓
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

✓
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 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]