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The mileage is about 30 thousand km, the engine type is G4FA, the transmission type is M5CF1-1, the 2012 Beijing Hyundai Rena 1.4 L manual block car. User response: the backing function of the car suddenly failed and could not work. Inspection analysis: the vehicle auxiliary system (RPAS) uses 4 radar probes to detect obstacles and communicate with the body control unit (BCM) through the LIN line. The BCM starts the buzzer or sends the display data according to the information transmitted by the radar probe. When the system is switched to the reverse gear, the system will be self checked first, if no fault is found, and the buzzer will send out a 0.3 s alarm sound after the power 0.5s. If a radar probe is found to fail, the fault location will be indicated by a different rhythm of alarm. For example, Zuo Houlei is a "beep, a beep, a beep", and the left center Radar is a "beep, a beep, a beep". The maintenance personnel inspection found that after hanging down the system, the system sent out the following alarm sound: "beep, beep, beep, beep, beep, beep, beep, beep, beep, beep, beep, beep, beep, beep and beep", which means that all 4 radar probes have questions. The maintenance personnel used stethoscope to listen to the radar probe, and there was no pulsing signal, indicating that the 4 radar probes really did not work. But there is no trauma at the back of the car, and the probability of simultaneous damage of the 4 radars is very low. Therefore, they should first check their common power and grounding. According to the circuit diagram, the FR21 connector No. 1 foot voltage is 12 V, and the 2 and 4 feet are all normal to the lap iron. The FR21 connector has no short circuit or circuit breakage to the wire harness of the radar probes. Next we have to consider whether the problem lies in the signal output or signal transmission of BCM. BCM communicate with each radar probe through the LIN line, and the LIN bus is a supplement to the CAN bus. It is only a serial communication system for single signal communication. Only one wire makes LIN communication a simple network. From the BCM to the radar probe, there is only one connector MF11 in the middle of the LIN line transmission. The maintenance personnel try to direct from the BCM's M04-B connector 15 foot to the 3 foot of the connector FR21, and the result is that the radar can work properly. The voltage of the jumper conductor is 10.94 V, while the original LIN line voltage is 11.85. The difference between the two is 0.91 V. Subsequently, the LIN line voltage of other normal vehicles was measured, confirming that the normal value should be about 10.9 V. It can be seen that there is a problem in this harness. Troubleshooting: replace floor wiring harness and troubleshooting. Review summary: the LIN line of the car has a 0.91 V voltage without any reason, but the dismantling of the line does not find external breakage, and because the harness is within the manufacturer's warranty, the old part needs to be returned to the manufacturer. We can not conduct a destructive inspection, only to infer that the cause may be internal adhesion or force extrusion of the wire harness. Cause LIN line and power line mutual inductance.