Symptoms of a bad upstream O2 sensor

[erode]

After I did my exhaust, my CEL was expected since I went catless. However, I ran the codes with the reader I bought and it showed Bank1 upstream O2. If I am not mistaken, this is the driver side O2 that is nearest to the exhaust manifold.

My understanding is that this one affects the air-fuel ratio and can really mess with power output, gas mileage. Correct? Could it put the car into a "limp mode"? It is difficult for me to tell if the car's power output is right or not. My 400hp M5 is significantly stronger but the M5 is 4025lbs and the B7 is pretty damn close to the same. Maybe the S4 will never pull as hard as that car, even though the gears are much taller than the M5. My shitty speculation abilities are just driving me up a wall and I wouldn't mind some input.

Oh, and I was quoted $460 parts & labor to install that thing. I would definitely do that myself having been up in there before without problems. Does that quote seem fair?

[mad70sx]

Hmmm...by "upstream" do you mean primary or secondary O2 sensor? I have also seen them referred to as pre(primary) and post(secondary) cat sensors as well. "Upstream" seems a bit vague.

The primary sensors are the sensors closest to the engine,the secondary are farthest from the engine. Primarys sensor factor into AF ratio but the secondary do not,I believe. Secondary are only there to monitor catalytic efficiency and health.

I'm not 100% certain about this. This is just what I've gathered from researching my own O2 sensor issue. Hopefully someone more knowledgeable on the subject can chime in.

[erode]

Hmmm...by "upstream" do you mean primary or secondary O2 sensor? I have also seen them referred to as pre(primary) and post(secondary) cat sensors as well. "Upstream" seems a bit vague.

The primary sensors are the sensors closest to the engine,the secondary are farthest from the engine. Primarys sensor factor into AF ratio but the secondary do not,I believe. Secondary are only there to monitor catalytic efficiency and health.

I'm not 100% certain about this. This is just what I've gathered from researching my own O2 sensor issue. Hopefully someone more knowledgeable on the subject can chime in.

Thanks for your reply. Upstream as in closer to the motor than the other sensor post-cat. Primary. This is still an assumption of mine because I don't know with certainty that there aren't any other O2 sensors on this car. If I'm wrong, please tell me.

If the primary O2 was bad, I feel like it would have a significant impact on performance and overall well-being of the engine. Something I intend to fix the moment I figure this bit out!

[mad70sx]

There are 4 O2 sensors,2 primary and 2 secondary.

If the primary O2's were bad,I'm pretty sure it could effect engine performance and well being. Especially it it were causing a lean condition and engine knock,which is definately bad for an engine. Do you hear any knock or ping when under load?(indicating a lean condition)

[erode]

No, if anything it seems rich.

[Omerta]

Best/crappist diagram I could find of the locations.

Did it throw a code for upstream and downstream (as expected from the mod)?

[erode]

Officially I can't remember the exact codes, but when I get home I will check it out. I think it was only upstream bank 1

[Jsmooth65]

upstream is the one before the pre-cat. I've always assumed that if one fails, it still runs smoothly because the computer relies on the other bank's readings, but will throw a CEL.

downstream or secondary o2 only measures a delta between the primary o2 to see if the pre-cats are there and working.

[erode]

Started the car up and ran the codes with a different tool (the code I mentioned came from my mechanic's tool) and I got this:


Current Fault Log
------------------
P0431: Warm Up Catalyst Efficiency Below Threshold (Bank 2)
P0421: Warm Up Catalyst Efficiency Below Threshold (Bank 1)

Which makes sense because I don't have cats. Not sure why if the other code exists at all or if it's just not there right now.