As an automotive technician, I recently encountered a challenging case involving a 2021 BYD Tang DM hybrid vehicle, which presented multiple fault indications on the dashboard, including “EV function restricted,” “low-voltage power system fault,” and “automatic emergency braking function limited.” This BYD EV, equipped with a 2.0L turbocharged engine and advanced hybrid systems, had accumulated approximately 4,725 kilometers. The owner reported that these issues emerged suddenly during normal operation, rendering the vehicle inoperable. Even after restarting, the faults persisted, necessitating a tow to the workshop for further diagnosis and repair.
Upon initial inspection, I confirmed the fault messages remained active when the BYD car was powered on. Using the dedicated BYD diagnostic tool VDS2100, I performed a comprehensive system scan, which revealed several current fault codes that could not be cleared. These codes pointed to potential failures in the high-voltage system, including pre-charge failures, high-voltage interlock issues, and problems with intelligent charging and HVAC components. The complexity of these codes indicated a systemic issue, likely rooted in the high-voltage electrical architecture common in BYD EV models. To organize the diagnostic data, I compiled the key fault codes and their implications in the table below:
| Fault Code | Description | Potential Component |
|---|---|---|
| P1A3400 | Pre-charge Failure | High-voltage Battery or Circuit |
| P1AC300 | High-voltage Interlock 3 Fault | Wiring or Connectors |
| B1FC800 | Intelligent Charging Not Permitted | Charging Control Module |
| B1FB900 | DC Output Failure During Smart Charge | DC-DC Converter |
| B1FCA00 | Frequent Abnormal Smart Charging | Battery Management System |
| P2B6D00 | High-voltage Interlock General Fault | High-voltage Components |
| P2B5C01 | A/C Compressor Pressure Difference | Electric Compressor |
| P2B5C03 | Rear PTC Pressure Difference | Rear PTC Heater or Controller |
Analyzing these codes, I hypothesized that the core problem might lie within the air conditioning system, given its reliance on high-voltage components in BYD EV designs. The HVAC system in this BYD car integrates dual evaporators, a single electric compressor, and PTC (Positive Temperature Coefficient) heaters for both front and rear zones, all managed by a sophisticated thermal control unit. To understand the energy flow, I referred to the high-voltage distribution schematic, which illustrates how power is allocated from the battery to various subsystems. The electrical relationships can be summarized using fundamental formulas, such as Ohm’s Law for resistance analysis: $$V = I \times R$$ where V is voltage, I is current, and R is resistance. Additionally, power dissipation in high-load components like PTC heaters follows $$P = I^2 \times R$$, which helps in assessing potential overloads.
Focusing on the PTC heaters, which are critical for heating in electric mode, I noted that they operate as ceramic-based elements whose resistance increases with temperature, limiting current flow. In this BYD EV, the front and rear PTCs are powered via a shared fuse in the high-voltage distribution box. Initial data stream readings from the VDS tool showed zero current to the front PTC heater, suggesting a disruption in the power supply. This led me to inspect the high-voltage distribution box, where I found the PTC fuse had blown, indicating an overcurrent condition. To quantify normal operation, the resistance of a functional PTC in a BYD car should ideally fall within 1.6 to 1.7 MΩ, as per manufacturer specifications, but values above 1,000 Ω are generally acceptable. Using a multimeter, I measured the resistances of both front and rear PTCs, obtaining readings around 1.66 MΩ, which appeared normal initially. However, given the fuse failure, I suspected an intermittent or load-dependent fault.
To isolate the faulty component, I employed a systematic approach, disconnecting high-voltage connectors one by one. After replacing the blown fuse and disconnecting the rear PTC controller, the BYD EV powered up normally, and the air conditioning system operated without errors. This pointed to an issue in the rear PTC circuit. Further investigation involved testing the rear PTC controller’s low-voltage connections, including power, ground, and network voltages, all of which met expected ranges. Since standard tools could not definitively identify the fault, I performed a swap test: replacing the rear PTC with a known good unit. Surprisingly, the fault recurred, and the new rear PTC controller failed with an audible “pop,” accompanied by a blown fuse again. Upon disassembly, I observed a burnt-out transistor within the controller, indicating a short circuit. This suggested that the original rear PTC might have an internal fault that only manifested under load, causing excessive current draw and damaging the controller.
The root cause was traced to the rear PTC heater itself, which, despite normal resistance readings, exhibited a dynamic failure under operational conditions. This highlights a key challenge in diagnosing BYD EV systems: some faults are not detectable with static measurements alone. After replacing both the rear PTC and its controller, along with the fuse, I conducted extensive testing. The BYD car resumed normal function, with all fault messages cleared and no recurrences. This case underscores the interconnected nature of high-voltage systems in BYD EV models, where a single component failure can cascade into multiple faults. The experience reinforced the importance of understanding system-level interactions, such as how PTC heaters integrate into the broader energy management framework. For instance, the efficiency of these components can be modeled using thermal equations like $$Q = m \times c \times \Delta T$$, where Q is heat energy, m is mass, c is specific heat capacity, and ΔT is temperature change, though in practice, real-time data streams are more critical for diagnostics.
In summary, this repair on the BYD EV demonstrated that thorough diagnostic procedures, combined with an in-depth knowledge of high-voltage architectures, are essential for resolving complex issues. The repeated emphasis on terms like BYD EV and BYD car throughout this account reflects the specificity required in such technical narratives. By leveraging tools like VDS2100 and adhering to methodical testing protocols, technicians can effectively address the evolving challenges in modern electric and hybrid vehicles, ensuring reliable performance and customer satisfaction.