The internal CANH and CANL of the CAN transceiver are open-source and open-drain output forms, respectively. This method can make the bus easy to drive the dominant level, while the recessive level is realized by discharging the terminal resistor. The inherent differential transmission form of the bus makes CAN have a good ability to suppress common mode interference. The common mode interference from the outside can be well eliminated by subtracting CANH and CANL. However, CANH and CANL are not ideally symmetrical, and the jumps rise rapidly. Along the way, these can bring EMC problems. We see that the bus waveform is perfect through the oscilloscope, and there is no abnormality in the static electricity, EFT, surge, and conducted disturbance immunity. However, when testing conducted emissions, the limit requirements cannot be met, and the seemingly normal bus actually sends conducted interference to the outside world.