(1) Fault symptoms: The chassis model of this car is E46, equipped 

with an M43 engine. The fault symptoms are as follows: The engine 

runs for 12 There are intermittent fluctuations from time to time. 

When starting in gear or turning on the air conditioning system, the 

engine speed fluctuations are more obvious. 

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 (2) Diagnosis and maintenance: Use the diagnostic instrument to 

perform self-diagnosis and query the fault information of the engine 

system. There is no fault code. A routine inspection was performed 

and no problems were found. Replace spark plugs and fuel filters, 

clean throttle valve. Check the idle solenoid valve and find that there

 is oil in its working chamber. After cleaning it, measure its 

resistance and it is 102 [standard value is (8±2)Ω]. Reinstalled the 

idle solenoid valve and tried driving, but the fault persisted. Use an 

oscilloscope to measure the control signal of the idle solenoid valve.

 When the engine speed is 800r/ min, the signal waveform is a 12V 

square wave; when the engine speed is 1200~1500r/min, the 12V 

square wave gradually turns into a straight line, indicating that the 

control signal is good. Check the air flow sensor and measure the 

voltage at its terminal 1, there is 5V. Check the grounding condition 

of terminal 5, it is normal. Terminal 2 is the signal terminal, measure 

its signal voltage. When the engine speed is 1200~1500r/min, the 

voltage is 2.3~2.5V; when accelerating, the voltage can increase 

accordingly, which is normal. Terminal 4 of the air flow sensor is the 

signal terminal of the intake air temperature sensor. Measure the 

resistance between terminals 4 and 5. It is 23002 at 20℃ and 9702 at 

50℃, which is normal. Continue to check the throttle position 

sensor, the installation position is normal. Measure the signal 

voltage, which is 0.5V when the throttle is fully closed and 4.8V 

when the throttle is fully open. Moreover, the signal voltage changes

 evenly during the process of opening the throttle, indicating that the

 throttle position sensor is normal. Lift the vehicle and check the 

oxygen sensor, which appears to be in good condition. Use a 

multimeter to measure the signal voltage and find that the change is 

very small. When the engine speed reaches more than 2500r/min, the

 signal voltage is still 0.55V, which is inconsistent with the standard

 value, indicating poor performance of the oxygen sensor. Replace 

the oxygen sensor and the fault is completely eliminated. 

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 (3) Summary The signal voltage of the oxygen sensor should change

 within the range of 0.1~0.9V, and the number of changes per minute

 should be more than 20 times. When the mixture concentration is 

low, the signal voltage should be lower than 0.45V; when the mixture 

concentration is rich, the signal voltage should be higher than 0.45V

. The cause of the fault in this case is that the oxygen sensor is aging, 

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the signal voltage is too low, and the number of changes does not 

meet the requirements. The engine control module determines that 

the mixture is too lean based on the wrong signal and performs 

enrichment control, resulting in incomplete combustion of the 

mixture and engine jitter. The particularity of this fault is that 

although the oxygen sensor signal is inaccurate, the signal voltage is 

still within the standard value range. Therefore, the engine control 

module cannot record the relevant fault code. This requires 

maintenance personnel to check item by item through conventional 

methods, making maintenance difficult. There has been an increase.