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A Chery QQO. 8 L car with a mileage of about 56 thousand km and a AMT transmission on 2007. Customer response: the vehicle warning light flickers, and the transmission can not be disconnected from the 2 gear. After the car: the test fault is as described by the customer. Generally, the main causes of the fault include: (1) the mechanical part of transmission is issued. (2) the fault of the speed selector. 3. The internal fault of the transmission control unit can not control the quick selector. Short circuit, circuit breaker and virtual connection of the related lines. First, the fault diagnosis instrument is connected to the vehicle to detect the vehicle. The equipment shows the meaning of the odd number solenoid valve short circuit, the open fault, the clutch position sensor short circuit, the friction disk speed sensor short circuit, the open circuit, the control unit system error and the ECU avalanche feed fault code. According to the above test results, there are so many faults in the system. There are several reasons: (1) there are failures in the connection or power supply of the relevant sensors. 2. The main power supply or the grounding of the transmission control unit is unstable. (3) the internal damage of the control unit of the transmission. For this reason, we first removed the fault code with the fault diagnosis instrument. After the operation is completed, it is found that all other fault codes are cleared, but the fault code, which means the odd number solenoid valve short circuit and the circuit breaker, can not be cleared, indicating that the fault code is a real fault. The fault code was analyzed, and the possible causes of the failure were as follows: 1. The opening or damage of the odd number solenoid valve. (2) the fault of the related line or connector. (3) the internal damage of the control unit of the transmission. In view of the failure of the customer's purchase of the car for second days, it is considered that the possibility of a quick selector is less likely to occur, and the problem is likely to be on the line. Due to the trouble of checking the circuit, it is decided to replace the transmission control unit to carry out the test. But after replacing the transmission control unit, the trial run is still in trouble. It is impossible to measure the transmission control unit and the odd electromagnetic valve line. Finally, after further inspection, it is determined that the brake pedal and transmission control line are broken.