1. Checking the resistance of the oxygen sensor heater
Disconnect the oxygen sensor wiring harness connector and use a multimeter in resistance mode to measure the resistance between the heater terminal and the ground terminal in the oxygen sensor wiring harness. The resistance should be 4-40Ω (refer to the specific vehicle model's manual). If it does not meet the standard, the oxygen sensor should be replaced.
2. Measurement of oxygen sensor feedback voltage
When measuring the feedback voltage of the oxygen sensor, disconnect the oxygen sensor wiring harness connector, refer to the vehicle's circuit diagram, and lead a thin wire from the oxygen sensor's feedback voltage output terminal. Then, reconnect the wiring harness connector and measure the feedback voltage from the lead wire while the engine is running. (For some models, the oxygen sensor's feedback voltage can also be measured from the fault detection socket. For example, the feedback voltage of the oxygen sensor in Toyota's series of cars can be directly measured from the OX1 or OX2 terminals in the fault detection socket.)
When testing the feedback voltage of an oxygen sensor, it is best to use a pointer-type multimeter with a low range (typically 2V) and high impedance (internal resistance greater than 10MΩ). The specific testing method is as follows:
1) Warm up the engine to normal operating temperature (or run it at 2500 rpm for 2 minutes after starting);
2) Connect the negative probe of the multimeter (voltage setting) to E1 in the fault detection socket or the negative terminal of the battery, and the positive probe to OX1 or OX2 in the fault detection socket, or to the output wire on the oxygen sensor harness connector;
3) Keep the engine running at approximately 2500 rpm while checking if the voltmeter needle swings back and forth between 0-1V. Record the number of swings within 10 seconds. Under normal circumstances, as feedback control proceeds, the oxygen sensor's feedback voltage will fluctuate around 0.45V, and the number of changes should be no less than 8 within 10 seconds. If it changes less than 8 times, it indicates that the oxygen sensor or feedback control system is malfunctioning. This may be due to carbon buildup on the oxygen sensor surface, reducing its sensitivity. To address this, run the engine at 2500 rpm for about 2 minutes to remove the carbon buildup from the oxygen sensor surface, and then check the feedback voltage again. If the voltmeter needle still changes slowly after removing the carbon buildup, it indicates that the oxygen sensor is damaged or there is a fault in the computer feedback control circuit.
4) Check the oxygen sensor for damage.
Disconnect the oxygen sensor wiring harness connector to remove the oxygen sensor from the computer, putting the feedback control system in open-loop control mode. Connect the positive probe of a multimeter (voltage setting) directly to the oxygen sensor feedback voltage output terminal, ensuring the negative probe is properly grounded. Measure the feedback voltage while the engine is running. First, disconnect the crankcase ventilation hose or other vacuum hose connected to the intake manifold to artificially create a lean mixture. Observe the voltmeter reading; it should decrease. Then, reconnect the disconnected hose and disconnect the coolant temperature sensor connector. Replace the coolant temperature sensor with a 4-8KΩ resistor to artificially create a rich mixture. Observe the voltmeter reading; it should increase. Alternatively, you can change the mixture concentration by suddenly pressing and releasing the accelerator pedal. When the accelerator pedal is suddenly pressed, the mixture becomes richer, and the feedback voltage should increase; when the accelerator pedal is suddenly released, the mixture becomes leaner, and the feedback voltage should decrease. If the oxygen sensor feedback voltage does not show these changes, the oxygen sensor is damaged.
Additionally, when testing a titanium dioxide oxygen sensor using the above method, a good oxygen sensor should have an output voltage that fluctuates around 2.5V. Otherwise, the sensor can be removed and exposed to air to cool before measuring its resistance. If the resistance is high, the sensor is good; otherwise, it should be replaced.
5) 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.
Faults can also be identified by observing the color of the tip of the oxygen sensor.
① Light gray tip: This is the normal color of the oxygen sensor;
② White tip: Caused by silicon contamination; the oxygen sensor must be replaced in this case. ③ Brown tip: Caused by lead contamination; if severe, the oxygen sensor must also be replaced.
④ Black tip: Caused by carbon buildup. After troubleshooting engine carbon buildup, the carbon buildup on the oxygen sensor can usually be cleared automatically.
