I. Function of Oxygen Sensor
In engines that use three-way catalytic converters to reduce exhaust pollution, oxygen sensors are essential components. Because the purification capacity of the three-way catalytic converter for CO, HC, and NOx drops sharply once the air-fuel ratio deviates from the stoichiometric ratio, an oxygen sensor is installed in the exhaust pipe to detect the oxygen concentration in the exhaust and send a feedback signal to the ECU. The ECU then controls the amount of fuel injected by the injectors to maintain the air-fuel ratio near the stoichiometric value.
The ECU determines whether the air-fuel ratio is too low or too high based on the difference in electromotive force from the oxygen sensor and controls the injection duration accordingly. However, if the oxygen sensor malfunctions and causes an abnormal output electromotive force, the ECU cannot accurately control the air-fuel ratio. Therefore, the oxygen sensor can also compensate for errors in the air-fuel ratio caused by wear and tear on mechanical components and other parts of the electronic fuel injection system. It can be said to be the only "intelligent" sensor in the electronic fuel injection system.
The sensor's function is to measure whether there is excess oxygen in the exhaust gas after engine combustion, i.e., the oxygen content, and convert the oxygen content into a voltage signal that is transmitted to the engine computer, enabling the engine to achieve closed-loop control with the excess air factor as the target; ensuring that the three-way catalytic converter has the maximum conversion efficiency for the three pollutants in the exhaust gas: hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx), and maximizing the conversion and purification of pollutants in the emissions.
II. Symptoms of a faulty oxygen sensor
1. Increased fuel consumption: Carbon buildup on the oxygen sensor can cause abnormal signal output, leading to an imbalance in the air-fuel mixture and thus increased fuel consumption.
2. Increased pollutant emissions will cause the vehicle to fail the annual vehicle inspection, resulting in failure to meet emissions standards. This is understandable, as while the three-way catalytic converter can purify exhaust gases, its purification capacity is limited. If the oxygen sensor malfunctions, incomplete combustion in the engine will lead to increased pollutant emissions, which cannot be mitigated even if the three-way catalytic converter is functioning normally. Therefore, if an older vehicle fails its annual emissions inspection, in addition to cleaning the three-way catalytic converter, cleaning or replacing the oxygen sensor should also be considered.
3. There is a possibility of black smoke. A malfunctioning oxygen sensor may cause an abnormal air-fuel ratio, resulting in black smoke.
4. Unstable engine speed and vibration are also caused by an abnormal air-fuel ratio.
III. Common Faults of Oxygen Sensors
1. Oxygen sensor poisoning
Oxygen sensor poisoning is a common and difficult-to-prevent malfunction, especially in cars that frequently use leaded gasoline. Even new oxygen sensors may only function for a few thousand kilometers. Mild lead poisoning can be eliminated by using a tank of unleaded gasoline, restoring the sensor to normal operation. However, excessively high exhaust temperatures often allow lead to penetrate the sensor's interior, hindering oxygen ion diffusion and causing the sensor to fail, in which case replacement is necessary.
In addition, silicon poisoning of oxygen sensors is also a common occurrence. Generally, silicon dioxide produced by the combustion of silicon compounds in gasoline and lubricating oil, as well as organosilicon gases emitted from improperly used silicone rubber gaskets, can all cause oxygen sensors to malfunction. Therefore, it is essential to use high-quality fuel and lubricating oil. During repairs, it is crucial to correctly select and install the rubber gaskets, and avoid applying solvents or anti-sticking agents to the sensor that are not specified by the manufacturer.
2. Carbon deposits
Poor engine combustion can lead to carbon buildup on the oxygen sensor's surface, or the entry of oil, dust, or other deposits inside the sensor. This can obstruct or block outside air from entering the sensor, causing inaccurate signals and preventing the ECU from promptly correcting the air-fuel ratio. Carbon buildup primarily manifests as increased fuel consumption and significantly higher emissions. Removing the deposits will restore normal operation.
3. Oxygen sensor ceramic fracture
The ceramic in oxygen sensors is hard but brittle; striking it with a hard object or blowing it with a strong airflow can cause it to crack and malfunction. Therefore, it must be handled with extreme care, and the sensor should be replaced immediately if any problems are found.
4. Heater resistance wire burns out
For heated oxygen sensors, if the heater resistance wire burns out, it becomes difficult for the sensor to reach its normal operating temperature and it will become ineffective.
5. The internal circuitry of the oxygen sensor is broken.
6. Inspection of the appearance and color of the oxygen sensor
Remove the oxygen sensor from the exhaust pipe and check the vent on the sensor housing for blockages and the ceramic core for damage. If damaged, replace the oxygen sensor.
