Does rear O2 sensor change engine management?

Post by **alschnertz** » 11 Jul 2020, 07:56

1995 854T, 259,997 miles

I am getting a code for catalytic converter efficiency and with this mileage and it once having bad valve seals, I'm not surprised it's shot. I don't have the $$ to fix it right now and want to just limp along for another few months / couple hundred miles.

My question is, does the rear O2 sensor make changes to the timing or fuel trim if it senses a bad catalytic converter?

I think I read somewhere that just the front O2 sensor controls all the engine management and that the rear O2 sensor simply looks for a change in the exhaust gases then trip the CEL if none is detected. However, I can't seem to find any information on whether the rear O2 sensor actually controls anything.

Thanks

Post by **erikv11** » 11 Jul 2020, 08:28

You have it right, the rear O2 does not control any engine parameters.

One way to deal with this would be to delete the rear O2 in TunerPro on a tuned ECU, FWIW.

Post by **alschnertz** » 11 Jul 2020, 11:10

erikv11 wrote: ↑11 Jul 2020, 08:28 You have it right, the rear O2 does not control any engine parameters.

One way to deal with this would be to delete the rear O2 in TunerPro on a tuned ECU, FWIW.

Ok, that's good.
I wonder if I got the rear sensor out of the exhaust stream (using an anti-fouler) the CEL would go out.

Post by **misha** » 11 Jul 2020, 16:42

Maybe it does not affect fuel trim on '95(M4.3 management)...but on my '97 with M4.4 it does.
Both o2 sensors are communicating with each other all the time and if rear one detects that something is wrong it will not just turn on CEL...it will also change fuel trim.

Post by **kallekula** » 11 Jul 2020, 16:59

alschnertz wrote: ↑11 Jul 2020, 11:10
erikv11 wrote: ↑11 Jul 2020, 08:28 You have it right, the rear O2 does not control any engine parameters.

One way to deal with this would be to delete the rear O2 in TunerPro on a tuned ECU, FWIW.
Ok, that's good.
I wonder if I got the rear sensor out of the exhaust stream (using an anti-fouler) the CEL would go out.

Yes it will but chances are smog inspection will see that and fail the car. They have mirrors they look with.

Post by **alschnertz** » 12 Jul 2020, 05:39

^ Good point. But I suppose it would fail anyway because of the bad cat.
Just trying to buy some time and eliminate the annoyance of the CEL.

Post by **erikv11** » 12 Jul 2020, 07:57

260k, that's no slouch of a '95!

If you send me a m4.4 turbo ECU I'll flash it up for m4.4 with no rear O2 and send it back. They are cheap at junkyards. You can try out m4.4 too, it has subtle improvements. Still not free I know but still one easy way to get the light turned off. And my flashing just sits here doing nothing ...

Also keep in mind the rear O2 itself may be what's bad or there is an exhaust leak, cat could be fine.

Post by **alschnertz** » 12 Jul 2020, 14:04

erikv11 wrote: ↑12 Jul 2020, 07:57260k, that's no slouch of a '95!

I'd say 260K isn't so bad either.
I swear this car has nine lives. One family owned (been a dd for 8 different family members!), has been threatened to be donated at least a half dozen times, but I can just never pull the trigger. I even posted just a few weeks ago wondering what I should pull before it goes away. Then don't you know, I have a breakdown with one of the other cars and have to pull this one from the weeds to use for the past week. Maybe I'll renew the registration in Sept. There's a ton of worn out wearable parts issues, but the sob will still just start up and go.
Photo is from 2 years ago when I was going to part it out. Still looks the same.

Thanks for the upgrade offer, but I'm fine with the way it is for now.

Post by **manovlov** » 13 Jul 2020, 08:56

the front panel is timeless...

Manov

callahanoffroad · Post by **callahanoffroad** » 14 Jul 2020, 05:25

alschnertz wrote: ↑12 Jul 2020, 14:04
erikv11 wrote: ↑12 Jul 2020, 07:57260k, that's no slouch of a '95!
I'd say 260K isn't so bad either.
I swear this car has nine lives. One family owned (been a dd for 8 different family members!), has been threatened to be donated at least a half dozen times, but I can just never pull the trigger. I even posted just a few weeks ago wondering what I should pull before it goes away. Then don't you know, I have a breakdown with one of the other cars and have to pull this one from the weeds to use for the past week. Maybe I'll renew the registration in Sept. There's a ton of worn out wearable parts issues, but the sob will still just start up and go.
Photo is from 2 years ago when I was going to part it out. Still looks the same.

Thanks for the upgrade offer, but I'm fine with the way it is for now.

Up on the wood it looks like a Volvo statue or a trophy. I like it.