A TOYOTA milled 2.5 car with a mileage of about 63 thousand km and automatic transmission 2009. The owner reflected that the car had been maintained several times in the 4S shop. After the fault lights were processed less than 100km, it was lit up again, and the factory looked for solutions. Using KT600 diagnostic apparatus, we found that P0420 three yuan catalytic efficiency failure. Ask the owner of the fault lights after other abnormalities, such as the power reduction and fuel consumption, the owner clearly expressed no other exception, because the owner claimed that in the 4S shop maintenance several times are cleaned out of the fault code after the 100km fault lights again, temporarily listed as difficult to deal with the obstacle. The spark plug was dismantled first, and it was found that it had good combustion, no carbon accumulation and no eating deviation. There was no leakage between the oxygen sensor and the post oxygen sensor, and the oxygen sensor data flow was checked. The change of the oxygen sensor was normal and the change of the post oxygen sensor was sometimes not good with the data of the front oxygen sensor. Initially identified as three yuan catalytic efficiency failure. As a result of three yuan catalytic price, try to play the bottle to clean, unfortunately, the warehouse only a bottle of three yuan cleaning liquid, according to the explanation, more than 2 of the volume of 2 bottles! Try to test whether a bottle can solve the problem with a try. After cleaning, give three fuel cleaners to the owner and tell them to fill a small bottle of fuel additives at each time. Because this time we used to use 2 three yuan cleaning agent, there is only one left. It may not be thoroughly cleaned.