The driving mileage is about 60 thousand km, the model is CA7204AT, the engine model is LF, the volume is 2. 0L, the equipment model is the FS5A - EL hand from the 5 speed transmission of the FAW Pentium car. The car has no response to R or D position. It must step on the accelerator pedal and get in the car to drive. The AT lights will be on when the hot car runs for a while. After the engine is switched off, restart, the AT lights go out and the vehicle can run normally. From 1 stalls to 5 stalls are normal, in the 4 or 5 gear deceleration, AT lights immediately lit, after the lights in the 3 stalls no longer jump. Fault diagnosis: fault code is detected by X431 diagnostic instrument: P0733 - 3 speed ratio is incorrect; P0841- oil pressure switch fault. Replacement of 1 hydraulic switches, 2 fault codes can be cleared, but after a while, there is still a fault code P0733. (1) analysis of the mechanism of the failure code of the mechanism of automatic transmission to 3 gear, TCM monitoring the speed of the forward clutch drum and the speed ratio of the differential (forward clutch drum speed / turbine signal; differential speed / speed signal). If the speed ratio is less than 0.063 or higher than 1.249, TCM finds that the 3 gear ratio is not correct. (2) analysis of the possible reasons: (1) the ATF level is too low or metamorphic; (2) the shift solenoid valve A, C stuck; (3) the pressure control solenoid valve absorption; (4) the pipeline pressure is too low; ((5) the 2-4 gear brake slip: 6. 3-4 gear clutch skidding; (3) road test to confirm the fault phenomenon before road test ATF, oil level and oil quality are all! The D position can be started, the 1 gear 2 gear has the impact, the 2 gear 3 gear has slight impact, the 3 gear 4 gear has slight impact, 4 gear 5 gear normal. In the trial run, from the 1 gear to the 5 gear, the AT light is not bright. When the 5 gear deceleration, the AT light starts to turn on. After the light is on, it does not jump in the 3 gear. After the engine is shut down, it starts again, the AT lights go out and the traffic returns to normal. According to the test condition, the fault may be out of the oil pressure, and then the basic inspection will be done. (4) inspection process and maintenance 1) measurement system oil pressure: pipeline pressure is 200kPa, normal pressure is 330 ~ 470kPa, R position pressure is 190kPa, normal pressure is 490 ~ 710kPa, D position pressure is 200kPa, normal pressure is 330 ~ 470kPa. 2) measurement of oil pressure analysis: (1) the low pressure of the pipeline indicates that the oil pump is not normal, and the wear or pressure regulating valve is not normal; the 02R position of the oil pressure can not explain the bad work of the main oil pressure regulating valve, not only the pressure but also the pressure relief; (3) the low pressure of the system is the low pressure of the system. 3) according to the results of low oil pressure analysis, dismantle the transmission oil pump and open the oil pump cover. The oil pump cover has scratches and dents in the inner rotor and the outer rotor. The wear is serious, and the new oil pump is changed. 4) while checking the oil pump, dismantling the transmission, checking the forward gear clutch, the 2-4 gear brake belt, the one-way clutch, the torque converter unidirectional clutch, the low speed reverse brake belt and the 3-4 gear clutch are normal, without abnormal wear and ablating. There is no problem in the mechanical system. 5) the wiring harness between the combination plug and the TCM plug on the transmission is checked. The turbine speed sensor, the speed sensor plug and the wire harness are normal, and the special harness in the transmission is normal. 6) change the oil pressure after the replacement of the new oil pump. The pressure of the pipe is 400kPa, the normal pressure is 330 ~ 470kPa, the R position pressure is 600kPa, the normal pressure is 490 to 710kPa, the D position pressure is 400kPa, the normal pressure is 330 ~ 470kPa. 7) oil pressure is normal, test car, from P displacement to R bit impact seriously, from P displacement to D bit impact than R bit light, speed up the gear normal, about 5km AT light is about, lock in the 3 gear can not jump gear, the speed lever hang to M potential drive, but dashboard does not display the number of M bit. 8) use X431 diagnostic instrument to detect, the fault code becomes F0734- four gear speed ratio is incorrect. Replacement of an input / turbine speed sensor, commissioning, failure has not changed, AT lights are still bright. 9) when the oil pressure is low, the gear can not run, and the time lag indicates that there is a problem of controlling the oil pressure. The oil pressure of the new oil pump is normal, but the traffic is serious. According to the analysis of the failure phenomenon, it is possible to control the valve problem, remove the oil bottom shell, the electricity check A, B, C, D, E shift solenoid valve have "clicking" work sound, the electromagnetic valve is normal. Disassemble the main body, find the pressure regulating valve card, with the screw screw to move, the force of the spring can not be back (normal valve mouth down, the stem can slide down by its own weight). Take out the pressure control stem, see the valve hole with pull marks, wear 1000 fine sand paper and put on the body test, the cold car hanging R position has a slight setback phenomenon, the hot car is normal, the AT lamp is not bright, the jump gear is normal, with the M position manual control increase and decrease gear are normal. Driver test car, can accept, take the car away. Fault summary: through the troubleshooting process of the car fault, it can be found that the root cause of the fault is that there is no timely replacement of ATF working fluid, too many worn metal particles, resulting in excessive wear and tear of the oil pump, discarding the valve holes and the valve body itself. The case reminds the people of 4S after sale service to be sure to notify the users to keep the maintenance in time so as to avoid unnecessary economic losses to the users, and then improve the reputation and popularity of the after-sales service of the 4S store.