What are the rear/downstream O2 readings supposed to look like?

[Cheburashka]

I am still chasing an issue with high Nox on my '95 Ranger 4.0

Thanks to RonD's help, I've deducted my EGR system is working nominally, so I am happy to get that out of the way. Unfortunately, I think this means there is an issue elsewhere. The worst part is the truck is running great, which makes this harder to diagnose.

I've moved on to the exhaust and O2 sensors. I am seeing some behavior from the downstream O2 that I do not expect.

First of all, PCM reports that downstream O2 heater is off. I thought it is always supposed to be on with a running car. Second of all, it appears to be very "lazy" to respond to throttle inputs. My understanding is it should switch from lean/rich at idle somewhat regularly and should be instantly reactive to switch to rich at wide-open throttle, which doesn't seem to be my case at all. It will switch to rich, but it takes a bit of time, and once I am off the throttle, it will stay rich for another 20 seconds before eventually switching to lean again. It's almost like it's slow to respond. I do not have any codes for the O2 sensors, but it is also not passing the catalyst self-test right now. Could this be a bad O2 sensor?

 

[Cheburashka]

Doing some more research, I actually don't think our trucks use the downstream O2 for anything other than monitoring the cat. Is this true?

In this case, I may need to check on my front O2 sensor operation somehow?

 

[19Walt93]

Doing some more research, I actually don't think our trucks use the downstream O2 for anything other than monitoring the cat. Is this true?

In this case, I may need to check on my front O2 sensor operation somehow?

You are correct, that's why they're Cat monitors and not downstream O2s. If memory serves, an O2 base number is 9F472 and a cat monitor is 9G444.

 

[Cheburashka]

Yeah, I guess it just happens the downstream cat heater is broken but that's probably not the source of my problems..