So i had recently done plugs and lubed the cops with die electric grease on my 01 4.6l, truck ran good for a little bit, then it was idling real low, re-did the battery relearn procedure and seems to be fine now so far. but i decided to hook up my scanner and just poke around and ran into this. Can anyone help me decipher what exactly this means? been googling but nothing i read seems real definitive and this seems odd since its holding steady. last summer if i remember correctly only one of these were reading at 99.2%. just need a nudge in the right direction as i am not familiar with this area.

 

Your looking at the rear cat monitor sensors for voltage.
It is also expressed in percent.
One side is well out of range.
If you do not have a code to support it like 420 or 430, you either leave it alone until there is a code or set about replacing the sensor or swapping the rear sensors to see if it is the sensor.
Otherwise it can be a sign the cats on that bank are nearing end of life.
Why; the computer looks at the switch ratios between the fronts and the rears.
When either these ratios fail, the code is set for whichever bank has failed.
This is how the system is able to tell when the cats are unable to clean the exhaust to a satisfactory degree.
The rear sensors are there to detect mostly oxygen after the cats do there job.
If their outputs back to the computer swing like the fronts then there is little 'ratio' between them.
This is what the program is designed to detect, as a failure.
Good luck.

 

hmm, i bet ive got a little of both failing. ive got 213k on my truck and judging by what ive seen ill bet the sensors and cats are original. it is rather doggy in my opinion and should be more peppy compared to my other friends' trucks. guess ill have to wait for a code to pop up some time. thanks for the info!

 

There's no such thing as a short term fuel trim for the downstream sensors. You're probably looking at a programming error, probably from an offshore programmer.

Ignore those S2 STFT readings, they're bogus.

Catalyst efficiency is measured by the ratio of downstream/upstream switching of the O2 sensors. You are not looking at that although you can see the immediate downstream O2 sensor voltages which, when looking at a single reading at at single point in time, is generally meaningless.