Two Oxygen Sensors Monitor a Catalytic Converter


Some vehicles have two oxygen sensors - one upstream and one downstream from the catalytic converter. This setup is supposed to monitor the efficiency of the converter, from what I have read.

How does this work? How would dual oxygen sensors monitor the converter?

Thank you.


All cars sold in the U.S, since model year 1996 are required to monitor the performance of the catalytic converter(s). It’s done using two oxygen sensors.

Catalytic converters begin their scrubbing of the exhaust stream by separating oxygen atoms from nitrogen oxide (NOx) molecules created during the combustion process. These freed oxygen atoms are then available for carbon monoxide molecules to grab to become carbon dioxide, and for unburned hydrocarbon molecules to split apart and bond to (the combustion process) in what’s called the “second burn”. However, they aren’t all used and the exhaust stream out of the catalytic converter should have more free oxygen atoms than the exhaust stream going into the cat converter. If there’s an insufficient improvement in the amount of oxygen between the upstream oxygen sensor and the downstream oxygen sensor, that means the converter isn’t functioning as effectively as it should.

The sensors themselves operate by comparing the oxygen in ambient air with the oxygen in the exhaust stream. A difference creates a voltage (or in some cases a change in resistance) between the surface of the ceramic detecting the ambient air and the surface sensing the exhaust stream.


The cat ‘burns’ the residual stuff coming out in the exhaust. Incompletely burned gasoline, CO and other stuff. This stuff passes over the catalyticly active surface of the cat and burns some more, not with an open flame, but still combining with the small amount of oxygen (O2) still in the exhaust. The first sensor measures the exhaust pre cat, the second sensor measures it post cat. There should be a reduction in the amount of O2 as the burning on the cat will consume O2. The cars computer monitors the difference between the two sensors and makes adjustments to the intake side (fuel/air mix) to keep the difference in spec. If it cannot keep them in spec, you will see the dreaded check engine light. Lots more going on than this, but this is an overview of how it works.



The upstream sensor . . . before the cat . . . should be fluctuating from below 200mv to above 800mv, with the average being around 450mv

The downstream sensor . . . after the cat . . . should NOT be wildly fluctuating. It should have a very steady signal. If it IS fluctuating at the same rate as the upstream sensor, the engine control module will interpret that as a cat that is not operating correctly, and you’ll get a P0420 fault code.

Lazy sensors, contaminated sensors, exhaust leaks, etc. must be taken into account when performing your diagnosis


What is the frequency of the fluctuation? Can you see with with an oscilloscope?


So guys, more oxygen upstream or downstream?



“What is the frequency of the fluctuation?”

The downstream sensor’s signal really should NOT be fluctuating. The upstream sensor’s signal should be fluctuating quite rapidly.

" . . . more oxygen upstream or downstream?"

Both. The pcm compares the signals from upstream and downstream sensor, and then decides if the cat is functioning correctly.

By the way, is there a reason why you’re asking for all these details? Do you have a car with a P0420 fault code . . . ?!


Yes, a friend of mine has a car with a P0420 code.

What is the best tool to observe a sensor’s fluctuation? An oscilloscope, a digital voltmeter, an analog voltmeter? An analog voltmeter might load the circuit too much.
That is why I would like to have a sense for the frequency. Is it tens or hundreds of hertz?

If you were to replace only one oxygen sensor, would you replace upstream or downstream and why?



“What is the best tool to observe a sensor’s fluctuation?”

A scan tool

A dmm is acceptable, but it wouldn’t really show fluctuations. A good one would show min, max, and average, though

An oscilloscope is usually quite pricey, and I wouldn’t recommend a DIY guy . . . I assume you fall under that category . . . to go to that much trouble and expense

I can’t advise you about replacing the sensors, because I’m not working on your friend’s car. That really needs to be diagnosed in person, not remotely over the internet



I have an oscilloscope, but not a scan tool.

What sort of an output would I get from a scan tool? What data would it show me?



A DMM won’t react quickly enough to read the voltage cross counts of the upstream O2 sensor.

The reason a scanner is used is because it’s easier to connect to the vehicle to get live data.

With an oscilloscope, you’ll have to connect directly to the signal out wire from the O2 sensor.

But an oscilloscope can be used. Set the scope up to read 0-1.0 VDC at a sample rate of .20 seconds.

Here are samples of what the signal should and shouldn’t look like.

The downstream O2 sensor signal should be flat-lined between .4-.8 VDC.

If there’s any cross counting occurring in the downstream O2 sensor signal, it indicates that catalyst efficiency is falling off.