In electronic fuel injection engines, the oxygen sensor used for closed-loop control of the fuel system is an important electronic component that monitors the oxygen content in the exhaust gas and feeds back a voltage signal to the ECU to control the air-fuel ratio at 14.7. At the same time, it also acts as an alarm component for various fault signals.

　　A common failure of zirconia sensors is that the surface is covered with lead or carbon compounds, causing gas to be unable to penetrate and oxygen ions to be unable to diffuse, resulting in failure. When the fault light is on and the sensor fault is read, it is necessary to diagnose it, because an oxygen sensor alarm does not necessarily mean that the sensor is faulty; its alarm signal is also affected by the following factors.

　　Ignition system operating condition; intake system sealing performance; exhaust system blockage; injector operating condition; fuel system oil pressure. Therefore, in engine maintenance, once an oxygen sensor alarm signal appears, a comprehensive analysis, judgment, and combination of computer and human brain should be used to diagnose the faulty part and perform reasonable repairs.

　　I. Oxygen Sensor Fault Diagnosis

　　From the characteristic curve of the zirconia sensor, it can be seen that: when the air-fuel ratio is maintained at 14.7, the signal reference voltage is 0.4-0.5V. When the air-fuel ratio is less than 14.7, the voltage gradually increases to 0.8-1V, indicating a rich mixture. When the air-fuel ratio is greater than 14.7, the voltage gradually decreases to about 0.2V, indicating a lean mixture. This is an important basis for oxygen sensor diagnosis. The diagnostic method is:

　　1. Run the engine at 2500 r/min for 2 minutes to preheat the sensor. Disconnect the sensor connector (for sensors with heating wires, pay attention to the connector position) and use a multimeter to measure the feedback voltage. Check the number of times the voltmeter pointer swings within 10 seconds. If it is less than 8 times, preheat the sensor again and check the number of times the pointer swings within 10 seconds. If it swings more than 8 times, it is normal. If it is still less than 8 times, proceed to the next step:

　　2. Disconnect the sensor harness connector and measure the feedback voltage.

　　(1) When it is greater than 0.45V, disconnect a vacuum tube somewhere on the intake pipe. If the voltage is still greater than 0.45V, the sensor is damaged. If it is less than 0.45V, the mixture is too rich, and the fuel, intake, or control system should be checked.

　　(2) When it is less than 0.45V, unplug the water temperature sensor connector and connect a 4-8KΩ resistor. If the voltage is still less than 0.45V, the sensor is damaged. If it is higher than 0.45V, the mixture is too lean.

　　II. Ignition System Operating Condition Check

　　Perform a routine check on the microcomputer-controlled ignition system with or without a distributor. The inspection items are spark energy, spark plugs, high-voltage wires, ignition timing, ignition advance angle, etc. When checking the ignition advance angle with an ignition timing light, the red clip is connected to the positive pole of the battery, the black clip is connected to the negative pole of the battery, and the high-voltage sensor clamps the high-voltage wire of one cylinder. The timing light is aligned with the ignition timing mark on the front pulley of the engine. When the engine speed increases, the ignition advance angle should increase. Hitting the knock sensor fixing screw or the cylinder head around with a hammer or wrench should significantly retard the ignition advance angle.

　　III. Intake System Sealing Performance Check

　　Connect a vacuum gauge to an appropriate location on the intake pipe. At engine idle speed (500-600 r/min), with sea level as the reference, the intake pipe vacuum should be in the range of 57.33-70.66 Kpa; otherwise, repair the intake system leak. At idle speed, the vacuum gauge pointer gradually drops to zero, indicating that the exhaust system is blocked. The change in the vacuum gauge pointer can also detect valve sealing and ignition performance.

　　Several Important Concepts Related to Oxygen Sensors

　　Detection Method: The oxygen sensor detects the concentration of the mixture, but it does not directly detect the mixture; instead, it detects the oxygen molecule content in the exhaust gas after the mixture is burned, thus indirectly obtaining the current concentration of the mixture.

　　Signal Characteristics: The oxygen sensor is actually a small battery with low voltage and low current. When the oxygen molecules on its inner and outer surfaces are at different angles, a potential difference is formed. Its outer surface extends into the exhaust pipe and directly contacts the engine exhaust, while its inner surface contacts the atmosphere, and the concentration of oxygen molecules in the atmosphere is constant.

　　The concentration of oxygen molecules in the exhaust gas changes with the change in the mixture concentration. When the actual air-fuel ratio of the mixture is higher than the theoretical air-fuel ratio (14.7, i.e., lean mixture), the concentration of remaining oxygen molecules in the exhaust gas is relatively high. At this time, the difference in oxygen molecule concentration between the inside and outside of the oxygen sensor is small, and only a voltage of about 0.1V can be output; while when the actual air-fuel ratio of the mixture is less than the theoretical air-fuel ratio (i.e., rich mixture), the remaining oxygen molecules in the exhaust gas are very few. At this time, the difference in oxygen molecule concentration between the inner and outer surfaces of the oxygen sensor is large, and a voltage of about 1.0V can be output.

　　Operating Characteristics: Most vehicle models currently use zirconia oxygen sensors. These sensors have an important technical indicator in their design, namely the signal rise time and fall time, both of which are required to be less than 250ms.

　　If this change time is greater than 250ms, although the voltage when the mixture is rich and the voltage when it is lean are sometimes normal, in practical applications, it manifests as a slow response of the oxygen signal, which cannot provide the engine computer with real-time mixture information, leading to malfunction of the fuel feedback system. Many soft faults are caused by this reason.

　　Control Principle: The engine computer uses the signal output by the oxygen sensor to understand the small deviation of the current mixture concentration relative to the theoretical value. Then, according to this signal, it adjusts the energization time of the injector accordingly to compensate for this small deviation, thereby improving the control accuracy. This is the so-called closed-loop control.

　　II. Common Faults of Oxygen Sensors

　　Heating element failure (generally open circuit);

　　Signal voltage is always at the lean voltage (failure);

　　Slow signal response (rich/lean voltage change time greater than 250ms).

　　III. General Detection Methods for Oxygen Sensors

　　1. Check the oxygen sensor heater resistance. Disconnect the oxygen sensor connector and use a multimeter's resistance range to measure the resistance between terminals 1 and 2 on the sensor side. The specific standard should be checked in the repair manual for the specific vehicle model, but generally, it should be between 4 and 40Ω. If it does not meet the standard value, the oxygen sensor should be replaced.

　　2. Check the oxygen sensor feedback voltage. Refer to the repair manual for the tested vehicle model to find the oxygen sensor signal wire, and insert the copper wire from the electrical wire into the corresponding operation's jack. Then plug in the connector, and use a multimeter's DC voltage range to measure the voltage between the copper wire and the negative pole. Note that a digital multimeter must be used, and the copper wire **** must not be grounded, otherwise the oxygen sensor will be irreversibly damaged. At this time, start the engine and make the water temperature reach at least 80℃. After making the engine reach 2500r/min multiple times, maintain the engine speed at 2500r/min and observe the voltage displayed on the multimeter. The voltage value should fluctuate rapidly between 0.1-1.0V. Within 10S, the voltage should change at least 8 times between 0.1-1.0V. If the voltage change is relatively slow, it is not necessarily a fault with the oxygen sensor or feedback control system; it may be that the oxygen sensor surface is covered with carbon deposits, leading to reduced sensitivity. In this case, run the engine at high speed for a few minutes to remove carbon deposits, and then observe whether the oxygen sensor signal voltage meets the specifications. If it still does not meet the specifications, proceed with **** characteristic analysis and inspection.

　　IV. Limitations of Traditional Detection Methods

　　For many years, most industry professionals have used the above methods for in-vehicle testing of oxygen sensors, or have used a good oxygen sensor for comparative testing, which can generally identify the fault. However, the above methods have some problems, and these factors often restrict the **** of detection:

　　1. The engine condition of the tested vehicle must be good to achieve the required test results. If the engine condition is poor, and the mixture is either rich or lean, the oxygen sensor cannot **** achieve changes between rich and lean signals according to the given test conditions;

　　2. The rich-lean voltage change time of the oxygen signal cannot be directly detected. Instead, it is indirectly assessed by the requirement that the signal voltage should change at least 8 times between 0.1-1.0V within 10S, which is highly arbitrary in practical applications;

　　3. As off-vehicle testing is not possible, the above methods have significant limitations and cannot effectively locate some more complex hidden faults;

　　【Kind Reminder】Anhui Tianfen Instrument Co., Ltd. is a domestic professional manufacturer of zirconium oxide oxygen analyzers. Our products are of reliable quality and have been recognized and used by numerous domestic power plants, textile factories, petrochemical plants, waste incineration plants, and other enterprises, achieving relatively ideal benefits. If you have any questions, you can consult customer service online via QQ, or call our hotline. We will have professionals available 24 hours a day to answer your questions in detail. We welcome your inquiries!

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