Lambda & O2 Testing — Simple is the easy way to get it right! (Don’t try to make things more complicated than they are).
Remember: The O2 sensor is a sensor, what it does is report only. It measures the Oxygen left after combustion in the emissions, it then reports back (to the ECU) its findings. That is it, that is all it does.
Don’t get side-tracked into thinking it adjusts anything, it doesn’t. The ECU acts on the information that the sensor sends to it, so if the fuelling is wrong it does not mean that the Lambda reading has to be incorrect.
Today by far the most common sensor used is a four wire, the sensor head is secured in the exhaust front pipe (with an air tight seal). The sensor head has a fly lead usually around 10” long; there is a multi plug at the end. The shape and design of the plug can vary from manufacturer to manufacturer, however the colour of the wires follow a standard configuration.
Pins 1 and 2 will have white wires; these are the wires for the heater within the sensor. The grey wire is an earth return; it is true that in all other cases within the industry we expect the earth wire to be black, but in this case we have to accept Grey. The Black wire is the signal wire, this is the wire you would wish to connect into when testing the sensor with an Oscilloscope.
Testing Lambda: Using an Oscilloscope back probe into Pin 4 (Black wire), this should give a waveform something like the one to the right with the engine hot and on Idle. This example shows the rich to lean switch, with rich to the top of the waveform and lean to the bottom.
Remember: This is showing you the sensor reporting back what it sees, this is not the switching actually being done. The switching is done by the ECU and is represented within a Scan Tool using live data. The line of data you would be looking at to get this information is Short Term Fuel Trim (STFT), you will also find the reading of Long Term Fuel Trim (LTFT) very useful as it will show you what has been happening in the longer term.
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