The car oxygen sensor uses the Nernst principle: the core element is a porous ZrO2 ceramic tube, which is a solid electrolyte, and porous platinum (Pt) electrodes are sintered on both sides. At a certain temperature, due to the different oxygen concentrations on both sides, oxygen molecules on the high concentration side (inside 4 of the ceramic tube) are adsorbed on the platinum electrode to combine with electrons (4e) to form oxygen ions O2-, making the electrode positively charged, O2 -The ions migrate to the low oxygen concentration side (exhaust gas side) through the oxygen ion vacancies in the electrolyte, so that the electrode is negatively charged, that is, a potential difference is generated.
What are the common faults of oxygen sensors
1. Oxygen sensor poisoning
Oxygen sensor poisoning is a failure that often occurs and is difficult to prevent. Especially for cars that often use leaded gasoline, even new oxygen sensors can only work for thousands of kilometers. If it is only mild lead poisoning, and then use a box of lead-free gasoline, the lead on the surface of the oxygen sensor can be eliminated and the normal operation can be restored. But often because of the excessively high exhaust temperature, lead invades into its interior, hinders the diffusion of oxygen ions, and makes the oxygen sensor ineffective. At this time, it can only be replaced.
In addition, silicon poisoning of oxygen sensors is also a common occurrence. Generally speaking, the silicon compound produced by the burning of silicon compounds in gasoline and lubricating oil, and the silicone rubber sealing gasket used inappropriately emits silicone gas, will make the oxygen sensor invalid, so use good quality fuel and lubricating oil . Rubber gaskets should be selected and installed correctly during repairs. Do not apply solvents and anti-sticking agents other than those specified by the manufacturer to the sensor.
2. Carbon deposit
Due to poor combustion of the engine, carbon deposits are formed on the surface of the oxygen sensor, or deposits such as oil or dust inside the oxygen sensor, which will hinder or block the outside air from entering the oxygen sensor, causing the signal output by the oxygen sensor to be inaccurate, and the ECU cannot be timely Correct the air-fuel ratio. The carbon deposition is mainly caused by the increase of fuel consumption and the significant increase of emission concentration. At this time, if the sediment is removed, normal work will resume.
3. Oxygen sensor ceramic chipping
The ceramic of the oxygen sensor is hard and brittle, and it can be broken and damaged by being hit with a hard object or blown with a strong air flow. Therefore, special care should be taken when handling, and timely replacement should be found if problems are found.
4. Heater resistance wire is blown
For the heated oxygen sensor, if the heater resistance wire is ablated, it is difficult to make the sensor reach the normal operating temperature and lose its function.
5. The internal circuit of the oxygen sensor is disconnected.
6 Check the appearance color of the oxygen sensor
Remove the oxygen sensor from the exhaust pipe and check whether the vent hole on the sensor housing is blocked or the ceramic core is damaged. If damaged, the oxygen sensor should be replaced.
What are the troubleshooting methods for oxygen sensors
1) Detect the resistance of the oxygen sensor heater: use an ohmmeter to measure the resistance between the oxygen sensor socket terminals (heating resistance). The colors of the two adjacent wires from the heating resistance are the same, which is very different. Cold resistance is about 4 ohms. (The oxygen sensor has a total of four wires, arranged in a row, one pair is connected, that is, the resistance end; the other pair is not connected, and the resistance end is not connected, which is the signal output end)
If it is detected as an open circuit or the resistance is not within the normal range, the oxygen sensor needs to be replaced; if the resistance value is normal, then proceed to the next troubleshooting.
2) Detect the power supply voltage of the oxygen sensor heater:
Turn on the ignition switch and measure the voltage between the terminals of the oxygen sensor plug (harness side) corresponding to the heating resistor end, which should be the battery voltage.
If the voltage is low or absent, repair the line from the oxygen sensor plug to the injection relay and ground.
3) Detect the insulation of the oxygen sensor resistance heater to the ground: use an ohmmeter to measure the resistance between the oxygen sensor resistance heater and the housing, which should be ∞.
If there is a passage, replace the oxygen sensor. If there is no passage, then proceed to the next inspection.
4) Check the signal voltage of the oxygen sensor:
① With the ignition switch off, disconnect the 4-pin connector on the oxygen sensor;
② Lead the 12V power supply of the battery to the resistance heating end of the oxygen sensor. This method needs to make a pair of wire connectors, that is, test tooling. After connecting, start the engine and measure the voltage at the signal output terminal after 2 minutes.
If you think that this method is not operable, you can start the engine directly. After 2 minutes, unplug the four-core connector and quickly measure the voltage of the oxygen sensor signal terminal. (After a long time, the core of the oxygen sensor will cool after the heating resistor is disconnected from the power supply, and the measurement error will increase) (I used the second kind)
In the idle state after starting the engine, according to the above working principle, this output voltage should be very low; then increase the throttle, there will be a voltage output at the moment of throttle change, this voltage is related to the rate of change of the throttle (that is, hold the throttle The voltage disappears immediately), the faster the voltage is, the greater. The maximum value can reach 0.9V. If it is a pointer meter, due to inertia and damping factors, this voltage can only be read to 0.8V. (Considering the response time of the digital table, it cannot be measured with a digital table, otherwise the error will be very large)
If there is no voltage output of the oxygen sensor, the voltage value is unchanged, the voltage rise or fall is very small, and the voltage change is very slow, it means that there is a problem with the sensing element of the oxygen sensor. At this time, consider cleaning the oxygen sensor.