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Last time we discussed using an oscilloscope to measure the signal of a car’s air flow meter; this time we will talk about using an oscilloscope to measure a car’s oxygen sensor. The oxygen sensor is also known as a λ (Lambda) sensor. Compared to the air flow meter sensor, although both affect the amount of fuel injected, their functions are different. The air flow meter is primarily used to control and measure the amount of air entering a car engine. The engine control unit uses this signal to determine the amount of fuel that needs to be injected, making it the key signal for calculating the fuel injection volume. The oxygen sensor is used to check the outcome of fuel injection by the injectors – whether too much or too little fuel has been injected. If too much fuel is injected, the computer will adjust the fuel amount accordingly to reduce it; if too little fuel is injected, the computer will increase the fuel amount. This is done primarily to reduce the engine’s emissions and prevent excessive pollution of the environment. Oxygen sensors in vehicles are generally divided into zirconium oxide and titanium oxide types. Zirconium dioxide oxygen sensors reflect changes in the concentration of the combustible mixture through voltage changes, while titanium dioxide oxygen sensors reflect such changes through resistance changes. Early oxygen sensors were heated by the exhaust gases, and such sensors had to wait a few minutes after the engine started running before they could begin to function. Today, most cars use oxygen sensors equipped with heaters; these sensors contain an electric heating element that allows them to reach their operating temperature quickly once the engine starts. Today, we will teach you how to use an oscilloscope to measure the oxygen sensor signal of a zirconium oxide strip heater. As shown in the figure below is an oxygen sensor with a heater: it can be seen that this oxygen sensor has four wires in total; the two black wires are the positive and negative terminals of the heater element, the blue wire carries the sensor signal, and the white wire serves as the sensor ground connection. Different manufacturers may vary; remember to consult relevant information when measuring. By using an oscilloscope for measurement, it is possible to check whether the heating element of the oxygen sensor and its operation under control by the engine control module (ECM) are functioning properly. Next, I will show you how to connect the oscilloscope: connect one of the oscilloscope’s channels to the current probe, and clip the current clamp around one of the black wires of the oxygen sensor in order to measure the current signal from the heater. Channel three is connected in the diagram. Connect the other channel of the oscilloscope to a BNC-to-banana plug cable, and then attach a probe and alligator clip; the alligator clip should be connected to ground, while the probe is used to detect the signal from the blue sensor wire. Channel two is connected in the diagram. Below is an example waveform: Channel 3 shows the current signal of the heater, which is a pulse width modulation (PWM) or square wave-type signal. As the temperature rises, the impedance of the heater increases. The voltage for the heater comes from the constant battery voltage supplied by the ECM (i.e., the value measured at channel 1 in the diagram); therefore, as the impedance of the heater increases, the current decreases. The most important characteristic of this waveform is not the height of the current pulses, but their width. The engine’s ECM controls the heater’s power by adjusting the width of each pulse. Channel 2 displays the voltage signal of the sensor, which represents the oxygen content in the exhaust gas. If you are using an ATO series oscilloscope, you can open the vehicle package and directly select the corresponding measurement item – sensor: oxygen sensor; Zirconium oxide (heated) – and the oscilloscope’s settings will be adjusted automatically.