Rich Lean switching for cat operation

Hi guys,
I am a newbie and was reading about the wide band AFR and the O2 sensors. Now, what I understood is that with the O2 sensors, ECU has no way of telling the degree of richness or leanness and so switching back and forth between rich and lean is the only way to be close to stoichiometric. AFR, owing to their wideband, can detect the richness or leanness and the ECU can thus precisely adjust the fuel, making their output steady.
Things get confusing when Catalytic converter comes in. In order for it to work properly, it must receive mixture close to stoichiometric that is alternating slightly between rich and lean. Cars with conventional O2 sensors fulfilled this condition. But for the cars with AFR sensor where the mixture is maintained stoichiometrically, how does the CAT then work. Now some websites mentioned that for the CAT to work, the ECU still gives rich/lean pulses but these are not detectable. . I have seen the Voltage and current plots of these AFR sensors and there is simply no switching pattern. If it is so, then how the upstream AFR compares with the downstream O2 to check the catalytic efficiency.

Thanks

What gets me is that, if the mixture is so precise and unchanging with a wide band AFR, then why with a faulty cat does the downstream zirconia 02 sensor switch rich to lean in the same fashion as a car fitted with an upstream and downstream zirconia 02?

Excellent questions, Reventon! I might have some answers.

What you read about the ECM still pulsing the mixture rich/lean with air/fuel ratio sensors is absolutely true. And it IS detectable, just not easily. On my Snap-On Modis Ultra, I have to change the scan data scales to 'zoom in' on the sensor signal, and allow the idle to stabilize so the mixture doesn't change dramatically. This is why you'll observe frequent downstream switching on a system with a failed catalyst. I'm trying to find an older capture I had of this off an Acura... :blush: I dig this up and post it.

Another way manufacturers will look for catalyst efficiency is active (maybe 'invasive' is a better word) testing. This is where the PCM will go into open loop and purposefully jam the mixture rich and lean rapidly, looking for the downstream to react. Here's an example from an '05 Nissan Altima with a P0420. The test starts just after frame 600:



When the PCM sees results like this, it knows with confidence that the cat is dead.

Don't forget about downstream fuel control, either. This is now a very popular method for improving catalyst performance across a wider range of driving conditions.

Tyler wrote: Excellent questions, Reventon! I might have some answers.

What you read about the ECM still pulsing the mixture rich/lean with air/fuel ratio sensors is absolutely true. And it IS detectable, just not easily. On my Snap-On Modis Ultra, I have to change the scan data scales to 'zoom in' on the sensor signal, and allow the idle to stabilize so the mixture doesn't change dramatically. This is why you'll observe frequent downstream switching on a system with a failed catalyst.

That answers my question! Thank you sir.

Noah wrote: That answers my question! Thank you sir.

No problem! I did more digging, couldn't find the Acura capture I was looking for, but I did find this off an '04 Honda Element. I was trying to get it to overheat at idle, but I was watching the air/fuel ratio sensor at the same time. You can see the AF LAMBDA moving around in small increments.

The plot you have shown explains it all. Thank you