TheBathTub

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.

RLXXI

Those are the catalyst monitors see where it says B1 S2 that means bank 1 sensor 2 you want to see a high % there as it shows no unburned fuel air/gas coming out.

STFT is mainly relevant for B1 S1 and B2 S1 and that will vary depending on how well the engine is running.

STFT = Short term fuel trim, is adjusted by the pcm to overcome small problems that may arise and usually keeps the engine running as close to stoic as possible, if it stays high the pcm will kick in LTFT to try and compensate and when that fails you get a check engine light and either a rich or lean code.

LTFT = Long term fuel trim. Both trims are a strategy programmed from the factory and you normally want to see them both near 0 as that means the pcm is not having to adjust or overcome a problem like a vacuum leak or other worn out/malfunctioning hardware. There again is mainly relevant for S1 both banks.

.

.

TheBathTub

so the 99.2% is normal as it is burning nearly all of the fuel like it is supposed to if im understanding you correctly?

RLXXI

Yep, not sure why they even have the stft listed for the down stream sensors, nothing the pcm can do to change that, only the chemical reaction of the cats internals can affect it.

Pull up just the voltages of all 4 sensors and watch them with the engine warmed up and in closed loop, both S1 sensors should be switching high to low over and over, that shows they are working, now watch the voltages of both S2 sensors, they should remain relatively steady and high indicating a large amount of oxygen coming out.

When the S2 sensors start reacting like the S1 sensors is when the cats have become chemically inert and need replacing.
.

Scott Baxter

Thank you for posting this. It is still helping 7 years later.

VladM

Thank you for your help. I ran a regular check on a truck I'm putting slowly together and I dropped my jaw when I saw that number. Now my jaw is back in place