MercedesBenz EQC400 new car air conditioning does not cool troubleshooting

Have you ever wondered why your car's air conditioning suddenly stops working? In this article, we’ll use a 2020 Mercedes-Benz EQC400 pure electric vehicle with only 20 km on the odometer as an example to explain how to handle AC issues discovered during a Pre-Delivery Inspection (PDI).

Vehicle Overview and Initial Symptoms

During the PDI, it was observed that the air conditioning system was not cooling, although the airflow adjustment worked normally. Additionally, a high-voltage system warning appeared on the dashboard during driving.

A quick diagnostic check with the Mercedes-Benz XENTRY system revealed fault codes in both the air conditioning control unit and the high-voltage battery control unit:

• U016B00 – Communication error with the electric air conditioning compressor

• P0E300 – Unknown fault code

Troubleshooting the Fault Codes

A guided test for the U016B00 fault code requested measuring the voltage between pin 1 and pin 3 of plug 1 on the compressor. According to the wiring diagram, pin 1 and pin 3 provide 12V power and ground to the compressor, while pin 2 is connected to the LIN bus for communication.

Measurements showed:

• Pin 1 to pin 3 voltage: ~12.3V (normal) – ruling out low-voltage power or ground issues

• Pin 2 to pin 3 voltage (LIN bus): 8.7V (normal)

Since LIN bus communication is more reliably checked with waveform analysis, the factory oscilloscope was connected to the LIN line. The results showed a stable 12V signal with a regular waveform, confirming that LIN bus communication was normal.

Inspecting the AC System

A visual inspection of the AC lines revealed no leaks. Refrigerant recovery and airtightness tests were normal. After refilling refrigerant, the AC still did not cool. This indicated a likely internal fault in the compressor.

It’s important to note that electric vehicle AC compressors receive both low-voltage and high-voltage power. While the low-voltage supply was verified as normal, the high-voltage supply from the battery pack must also be checked. The vehicle displayed a high-voltage warning, suggesting a possible connection to the AC failure.

High-Voltage System Check

The P0E300 fault code did not provide clear guidance. Measuring the actual high-voltage values showed only 2.8V at the high-voltage supply line, far below the normal range of 300–420V. This indicated a high-voltage fuse had likely blown.

Further investigation revealed that the AC compressor and associated components are protected by the same high-voltage fuse (F34/6f1). Measurement confirmed the 80A fuse was blown. Other high-voltage circuits were tested and no short circuits were found.

Resolution

Replacing the blown high-voltage fuse restored the high-voltage supply. After a test drive, the air conditioning system cooled properly, fully resolving the fault.

Maintenance Takeaways

Pure electric vehicles differ significantly from traditional fuel vehicles because they replace the internal combustion engine with electric drive systems. For maintenance technicians, this requires a thorough understanding of the “three-electric” systems (battery, motor, and inverter) and the ability to correlate fault codes with observed symptoms.

In this case, peripheral factors could easily mislead diagnosis. It is crucial not to overlook the high-voltage supply to electric components such as the AC compressor and its connection to high-voltage alerts on the dashboard. Proper analysis and step-by-step verification can help quickly identify the root cause and prevent unnecessary replacement of components.

Conclusion

If your car’s air conditioning stops working, especially in a new electric vehicle, consider both low-voltage and high-voltage systems in your troubleshooting process. Understanding the interplay between fault codes, wiring