What does the secondary O2 sensor really do?

[jmmxc]

I have seen on the web experimental stories of people using an electronic gadget to feed a signal to the ECU port in place of the downstream O2 sensor. I do not know myself whether it is a common practice.
Is there any clearance on the other side, there is usually a port hole opposite?
jmmxc.

[D_V_ENT850R]

The secondary only monitors cat efficiency. On the slight occasion the primary O2 is bad the ECU may use it for slight changes. The ECU will "default" if it has an issue with the first O2! I have a wideband hooked up and the signal going to my ECU input for my primary sensor. No cat and no codes yet running with the wideband. It would set every week or so but it's been almost 3 weeks no light!
In my tune I just got there is a Cat delete and primary O2 sensor heater delete. Basically the ECU does not look for those values and will not set a check engine light! Very nice!

[jmmxc]

Good trick, it sounds like!
But, how about the efficiency of the overall system. Do you know if the engine is running lean or rich?
How do you deal emissions testing?
jmmxc

[D_V_ENT850R]

Here in the great state of FL we have no emissions testing! Plus a lot of emissions tests are done Via the DLC do as long as no codes are popping up in the ECU you're fine! My wideband also is a gauge so I can see exactly what the ECU is seeing! The tune is great with the things you can adjust and turn off it's good!

[jmmxc]

In my tune I just got there is a Cat delete and primary O2 sensor heater delete...

[quoteMy wideband also is a gauge so I can see exactly what the ECU is seeing....][/quote]
Oh.. I may have missed 2 things: you mentioned "tune" and wideband
Can you clarify what they mean in the context of the orignal question by pinky33.
Does the wideband relate to a type of O2 sensor?

Thanks,
jmmxc

[D_V_ENT850R]

Tune is a rewritten program for your ECU that will get the most out of the engine and it's abilities! They make adjustments to the (MAP), aka your fuel trim tables that are preset by the manufacturer! Different ones around IPD, Rica/Snabb, ARD, and maybe others! Starting around $400-$1000 depending on what you're looking for!

Wideband is a different O2 setup! The stock O2's operate on a "narrow" band. Now "just for examples, not actual," say your stock O2 will sample your A/F ratio, (a/f= air/fuel), every 2-3 minutes a wideband is almost if not exactly real time. You can set it up to send the input to your ECU. The end result is your ECU can adjust fuel trims faster! When modding your car especially if it's "forced induction," turbo or supercharged, you want to make sure you don't run too lean! Too lean can burn or even melt your pistons. Too rich is just a waste of fuel and no gains. 14.7 is what they call stoichiometric that is the perfect A/F ratio. At hot idle around that is what you want mine bounces around 14.4-14.9! At a long throttle run you do not want to see more than 12.4 and no lower than 10.8! The O2 sensors read oxygen levels in the exhaust. On a wideband the higher the number the more oxygen in the exhaust (lean), the lower the number the less oxygen in the exhaust (rich.) When a tuner puts a car on a dyno to tune it they watch the A/F ratio to keep it safe but get the most out of your setup! A little technical but that's as simple as I can put it out. If more clarity is needed I will try my best and there maybe some more input around! Hope that answers the questions you had!

[jmmxc]

Yes, I have read some technical articles about this subject some time ago, and it all makes sense to me.
In my case, the technology is of great interest to me, even if I am not at the point of trying it out. However I am interested at being able to monitor the electronic and the mechanical interactions that result in the car running as efficiently as possible. It makes great economic sense.
So, do you use a special scanner, display module etc... to monitor the vital signs of the system? Do you have something on board that gives instant (live) feedback of the engine conditions? Or is it simply the reprogram of the ECU?
Thanks for your explanation.
jmmxc

[aisrael]

Believe it or not, changing out my secondary o2 straightened her out. She performs as she should and I'm still surprised to say the least. Like I said earlier, I could find nothing else wrong with her and the primary o2 is less than a year old.

The primary o2 sensor is the direct feedback of the O2 content in the exhaust in a gas engine. If the ECM does not believe the signal from the front sensor, the ECM operates in the predetermined tables (limp home mode). The engine will run rich and consume more fuel with less power.

The rear O2 sensor does two things. It is used to measure the oxygen storage of the catalyst durring a predetermined mode of operation (every MFR is a little different). The second job of the rear O2 sensor is to give feedback to the engine of the overall AFR of the system for minimizing the tailpipe emissions. This sensor usually has a stedy voltage output (such as 0.5 V).

if the rear O2 sensor fails, the engine will run normally but will illuminate the check engine light and will fail the state inspection.

Aaron