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A TOYOTA Landa with a distance of about 50 thousand km. Customer response: when the vehicle is in service, the engine fault lights and sideslip lights suddenly appear when traveling more than 100 kilometers. Fault diagnosis: the read fault code is P0420 (the efficiency of the catalyst system is lower than the lower limit of B1), and the value of 02SB 1S2 is 0.13V, 0.09V, 0.58V, 0.60V, 0.68V, and numerical anomalies, while other air fuel ratio and oxygen sensor are normal. There is no trouble before the car is maintained, and the malfunction occurs after maintenance. At that time, the car did the throttle and the injector cleaning, so it was judged that the cleaning agent remained in the three element catalyst, causing the abnormal signal of the oxygen sensor, which caused the error of ECM to cause the engine trouble light to light. When the fault code is cleared, the vehicle will turn in. When the vehicle runs more than 100 kilometers, the engine fault lights are lighted again. The fault code is still P0420 (the efficiency of the catalyst system is lower than the lower limit of B1). To solve this problem, first we need to understand that the P0420 (catalyst system efficiency below the lower limit of B1) means that ECM is used to monitor its efficiency by installing sensors in front and rear of the three element catalyst (TWC). The first sensor, the air-fuel ratio sensor, sends the information before ECM to the catalytic process. The second sensor, the heating oxygen sensor, sends the information after catalytic treatment to ECM. ECM calculates the oxygen storage capacity of the three way catalytic converter and detects any aging in the three way catalytic converter. If aging occurs, ECM will light up MIL and set DTC. To understand the failure mechanism of P0420, we can analyze the fault according to the fault of the car. Because the car is the fault after cleaning the throttle and the injector, the aging of the three element catalyst is not possible, and the numerical change of the air fuel ratio and the oxygen sensor is emphasized. Using the active test, it is found that the value of 02SBlS2 is changed from 0.05V to 0.89V or from 0.89V to 0.05V in 2S when the fuel injection is increased (24.5% ~ 12.5%), and the frequency is too fast (the normal change in 10s). Oxygen sensor should be no exception. Is it three yuan catalytic converter aging? To remove the oxygen sensor fault, replace an oxygen sensor of a normal vehicle, and use the active test to increase the amount of fuel injection (- 24.5% ~ 12.5%) to reduce the fuel injection amount of 02SBIS2 is still in 2S from 0.05V to 0.89V or from 0.89V to 0.05V. There is no change in the numerical value, the exhaust system has no gas leakage, the air fuel ratio is normal, and oxygen sensor monitoring oxygen content changes with the fuel increase and decrease instantaneously, indicating that TWC does not play the purification function B confirm the fault in the TWC. Confirm to the fault location, re analysis the problem, this fault is after the cleaning of the throttle and the injector, it is impossible for three yuan catalyst to be so fast aging, will be too much carbon in the inlet and cylinder, after cleaning the carbon covered on the three element catalyst surface, resulting in the low efficiency of the three catalyst, resulting in the misjudgement of the ECM. Clean the three way catalytic converter. The test failed for a week. Fault summary: the maintenance mentality is clear. Technicians use the function of the diagnostic instrument to solve problems quickly and avoid customers' complaints.