Fault diagnosis: 1) according to the failure conditions, the possible reasons are as follows: (1) IACV control valve failure. The fuel pump is inadequately supplied and the oil pressure is unstable. (3) the fault of the injector. The air intake system is leaking. PCM control system malfunction. 2) according to the trial of the special shop and the feedback of the driver, the special shop first cleaned the throttle body and started the engine according to its own maintenance experience, and found that it was still a small car at idle speed. 3) then according to the driver reflecting engine failure, Ding was lighted, so DTC read with HDS, and found the following two fault codes: P0134: A/F sensor heating system fault, P01157; A/F sensor AFS + /AFS- line voltage too high. 4) DTC scavenging with HDS, after scavenging fault code did not appear again, extract data table, from the waveform diagram can see the following data is abnormal: (1) the maximum value of the idle time space-time combustion ratio sensor reached 2.99 mA, the hourly -3. 87mA, alternately changing, and the normal value should float on the left right of 0mA and change very small. (2) the lower limit of air-fuel ratio is 0.69, the upper limit is 2, and the normal value is about 1. 3. ST fuel adjustment will change rapidly between 0.82 and 1.47. The data show that PCM can adjust the injection pulse width continuously according to the data of the air fuel ratio sensor, which makes the idle speed travel at idle speed. Based on these data, the air fuel ratio sensor is replaced. After the A/F sensor is dismantled, it is found to be a auxiliary plant. After the replacement, the idle speed is restored to normal, then the data sheet is extracted and the A/F sensor is observed. The value of the device is about 0mA, and the air-fuel ratio also reaches about 1, and the ST fuel adjustment is restored to 1.01. 5) replace the A/F sensor, the fault is completely eliminated. Maintenance summary: the A/F sensor plays a vital role in the fuel control system. If the A/F sensor is completely broken, the PCM will judge the A/F sensor fault, but if the A/F sensor only has the performance deviation, if the oxygen content in the exhaust gas can not be detected accurately, the PCM root causes the wrong instruction according to the wrong information. So more problems arise. It is very important to understand the parameters in HDS, which requires us to accumulate more studies at ordinary times.