If there is a telephone in the equipment room, find the two signal lines of the telephone in the original equipment room (the back of the telephone in the equipment room is usually marked with L and R), disconnect the two lines L and R, and use the voltage file of the multimeter to measure whether the two signal lines are live. If not charged, normal;

If there is a live case, it is necessary to check the two accompanying cable lines (straight to the bottom of the car), check whether the middle of the multi-channel transfer or other lines in parallel (see the drawing), after the check, disconnect the accompanying cable power supply source (until the two lines are completely independent), measurement without voltage, before proceeding to the next step.