I've just got a OBDII scanner and I've been playing around with it. I've been researching for hours about the 02 sensor live data and what I should be looking for. When I look at the screen it gives me a reading like .xx (v) and x% So I understand what the voltage means... but I don't get the percentage.
Why do the upstream sensors have the up and down "cycle" and the downstream ones stay fairly stable?

Basically the question is... When I'm looking at the 02 sensor live data... what exactly should I be looking for and what does it mean? (I know a good chunk of the answer but I want to hear it as if I don't know anything about it)

I know that it should also be different during idle and like say 2000 rpm.

I've also been researching about fuel trim. My one question is when I'm looking at the fuel trim live data... are there any numbers that would be a signal that my car isn't working properly? (like... x percent for a long period of time would mean something is wrong)

Thanks for listening!

 

The voltage of properly working upstream sensors continuously varies up & down as the computer adjusts the A/F mixture from lean to rich looking for the stoichiometric value. The downstream sensors hold a steady voltage because the O2 output a good working cat never changes. If the downstream sensors follow the upstream sensors, the cat is not working properly.

The PCM uses two fuel trim tables, the short term table changes with the output of the upstream sensors. After a short interval the short term table values are loaded into the long term table, and the short term trim is reset to mid value, and the cylcle repeats. There is a limted amount of (256?) values in the table can adjust to. Once the limit is reached the MIL lamp is lit and the ECM will complain about the A/F mixture being rich or lean.

As long as the PCM can adjust the A/F mixture, the trim values are of no consequece as how the engine runs.

At WOT the PCM swiches to open loop and uses the long term trim values.
Various other sensors, such as the MAP, MAF, ECT, IAT are use to adjust the short term trim.

 

Ok thanks. Is it normal for the two downstream sensors to be really different? One time I looked and saw the left side at .4x and the right side was maybe .7x. That's not always how it is but that's just one example. Is that ok? Would a good working cat yield a number that's higher or lower? And the downstream sensors change throughout driving... not alot... but they change. Is thaaat ok?

Thanks!

 

The purpose of the downstream sensors is to check the efficiency of the cat converter. If the converter is working properly the downstream sensor voltage will always be different from the upstream sensors and their output will be fairly level. If the downstream sensors track with the upstream sensors, the cat is not doing its job. Efficiency, in this case, means how good the cat is eliminating pollutants, not how good it is flowing.