I have a 2001 Honda Prelude SH (Honda H22A4 Engine).

The exhaust has two oxygen sensors; one before the catalytic converter (sensor 1) and one after (sensor 2). Holding at 2500 RPM and sitting still I see:

enter image description here

According to https://www.hondata.com/index.php?route=tech-closed-loop-oxygen-sensors:

OBD II engines use one oxygen sensor before the catalytic converter, and one oxygen sensor after the catalytic converter. The function of the second oxygen sensor is to determine if the catalytic converter is functioning. It does this by looking at the difference between the two oxygen sensors. If the catalytic converter is functioning correctly there will be a reduction in the exhaust oxygen content as carbon monoxide and carbon dioxide is catalyzed in the converter.

And my understanding is higher voltage = less oxygen.

So my question is simple: Since #1 (pink) is before the converter, and #2 (blue) is after, the higher voltage on #2 means an oxygen reduction in the converter which means everything in this graph is exactly what I'd expect to see with properly functioning sensors and a catalytic converter. Right?

I know it's a pretty straightforward question, I am just looking for confirmation that my understanding is correct.

1 Answer 1


While you have a general grasp of what is happening, the information is rather incomplete.

First, a catalytic converter does absolutely nothing with carbon dioxide (CO2). The CO2 just passes through. The converter works on carbon monoxide (CO), oxides of nitrogen (NOx) and hydrocarbons (HCx).

Second, one of the reasons the fuel mixture toggles back and forth (rich to lean and back) is to help the catalytic converter do it's job. When the fuel mixture is lean (excess oxygen) the converter consumes hydrocarbons and carbon monoxide by adding oxygen to them and stores oxygen inside.

When the mixture is rich (lacking in oxygen) the converter consumes NOx by stealing its oxygen and combining it with other things and uses the stored oxygen to convert hydrocarbons and carbon monoxide.

While it's true that this process gives a net reduction in exhaust oxygen content, this does not guarantee a good catalytic converter. This is because when the computer checks the converter it is looking for the oxygen storage element (cerium) to be functional. The function of the oxygen storage element can not be judge by passively comparing the two oxygen sensors. The computer most commonly does this with an active test by inducing a light misfire and monitoring the sensors.

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