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What Does The Rear O2 Sensor Do?

Let’s Talk About The Rear O2 Sensor

Is the critical unit is the upstream O2 sensor for emissions inspection? And what are the clear indications (based on scanner data) of a bad catalytic convertor, and by extension a failing rear O2 sensor?

The secondary sensor is purely to monitor the efficiency of the catalytic converter and be a tattle tale if the converter isn’t up to snuff.

Using an Oscilloscope to Monitor a Catalytic Convertor

The wave forms before cat should move in a up and down motions(hills/valleys) and after cat the wave form should be more leveled out if the cat is doing it’s job. Watching the voltage on a real time scanner you should see the voltage going up to around .800 Volts or 800mv and then down to .200 Volts or 200 mv. After cat the voltage should be somewhere in the middle of those numbers with the high/low peaks closer together or kinda like a straight line.

Using a digi high impedance voltmeter you connect the red lead to o2 sensor signal wire. With engine running in closed loop the voltage should read over 800mV and lower then 200mV and constantly fluctuate. Post cat O2 behind the cat should read be fairly constant, if the catalytic converter working properly. With that being said, normally the pre cat o2 is more important but the after cat O2 could also fail an emissions inspection (DTC).

What does the secondary O2 sensor really do?

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1 Comment

Not completely true; it plays a role in air-fuel ratio control. The total control authority is perhaps 2% but it is responsible for raising the catalyst efficiency from perhaps 75% to 98%. This is because in the precat position a lambda sensor has an accuracy of about 0.02 lambda. In the postcat position it has an accuracy of about 0.0005 lambda.

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