Can you have a failing o2 sensor with no check engine light?

[1 SX]

Also i should i looked at the values before i changed O2 sensor, but how do these look? I should get a better scan tool i know lol.

The volts readings change instantly on O2S11 & O2S12 now, where i'd assume the O2S12 was slow?

Also these readings are @ 2,000 RPMS.

 

[SHOZ]

A good sensor cycles three times per second. Not many readers can keep up unless you data log and then a cheap interface still cripples it.

So O2S1 should be cycling between a minimum of .005v and around a max of .750v. If it's stuck or doesn't go to the extremes then it's slowing down.

The O2S2 should be fairly stable when cruising in closed loop at constant throttle of around 90% -95% of your peak O2S1 voltage.

 

[gathermewool]

What were your fuel trims before and what are they now?

 

[1 SX]

Hi again, to update on this, New o2 sensor a couple hundred miles ago, and i gained almost 9 MPG!. No difference in driving style, or fuel. Car even feels more "peppy" under light load.
Before 25.6-26.2 MPG never above that, now its above 34 MPG. (parked it tonight at 34.9)

 

[1 SX]

the downstream doesn't affect engine running, it's just checking the cat is there and working

Maybe on some vehicles, but with this hyundai, it most definitely affected the MPG, and even throttle response.

 

[SammyChevelleTypeS3]

I think i get the picture here haha!, Im going to order a new sensor, and change it out today!. Car has 114.5K and has original sensor.
Thank you all!, Will update when new sensor is in.

Hope u not sorry you did not use anti seize.

 

[SHOZ]

You can actually remove the second O2 if you can live with the light. Some tuning programs eliminate the light for the lack of a post cat O2. Still have emissions not ready though.

 

[Thermo1223]

Yes, had a bad Wideband Upstream O2 on the old Forester and it reeked havoc with HP & drivability. Never flashed a CEL just was lazy. Once replaced it was night & day, drove like a brand new car. It was always fairly weather dependent.

 

[mightymousetech]

the downstream doesn't affect engine running, it's just checking the cat is there and working

On BMW they absolutely do. The DME uses info from both to calibrate the output from the primary.

Even on Honda. Had a CRV with a bad secondary O2. You could clear the faults, start it up, and it would start to run so rich the engine would stall after about 5 mins. Replacing the secondary O2 completely fixed it.

 

[michaelluscher]

Yes

 

[Jetronic]

On BMW they absolutely do. The DME uses info from both to calibrate the output from the primary.

Even on Honda. Had a CRV with a bad secondary O2. You could clear the faults, start it up, and it would start to run so rich the engine would stall after about 5 mins. Replacing the secondary O2 completely fixed it.

could be a nox prevention measure?

 

[sk_pete]

the downstream doesn't affect engine running, it's just checking the cat is there and working

imho first few inches of cat walls should catch most oil ash. bottom sensor should be almost clean .

 

[SHOZ]

If they do it's a pretty stupid strategy, IMHO, meant to prolong 'usefulness' of the cat at the expense, strictly an emissions output goal? Seems performance and optimum engine conditions come second place.

I can attest though that a Hyundai will run great without a post cat O2. And on a Hyundai the primary O2 failure will trigger a DTC but it has to be dead, dead, dead.....