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View Full Version : Second O2 seonsor. What does it do??



Blaze
04-04-2004, 10:22 PM
Hey Guys:

I’d like to better understand how the computer interprets the signal from the second O2 sensor. Specifically, I’m looking for techies with factual information, not hearsay or hypothesis.

Sorry if that sounds harsh, I don’t mean it to be. I’ve been asking a lot of questions on this subject and the answers have been all over the board.

I’m considering getting a cat delete pipe with an O2 sensor emulator chip. From what I have gathered, this chip will send a static signal back to the ECU that everything is AOK (ie no need to make any corrections to the fuel curve).

I thought that sounded pretty dangerous. My understanding is that the second sensor’s readings are evaluated against the first in order for the computer to make appropriate fuel corrections. Tampering with the sensors might cause any number of issues; generally all bad.

However, the other day, I was speaking to a tuner from a local shop who assured me that the car only requires the reading from the first O2 sensor to make it’s fueling decisions. The second sensor is not really involved. He further stated that the only reason the second sensor is there is to verify that the cat is doing its job. So if the computer gets a reading from a chip that everything is good…no harm, no foul.

So the question I have again is: ‘What is the computer doing with the second O2 sensor readings?’

Thanks in advance.

Jason

Matt
04-04-2004, 10:41 PM
Your tunner was right on the subject. The point of the secound o2 sensor in an obd2 equiped car is to assure the cat is doing its job properly

What happens with the secound o2 sensor is that if it reads out of its correct values for a eco friendly emmisions it will lean the computer out so hat there are less contaminates being expeled into the air. so the only time the secound 02 would have any effect or the car would be a cat delete scenario and it would lean the motor out and cause problem. So get the o2 simulator chip you were speaking of and you will have no problem.


Sorry if this sounds a little out of wack. its 1:30 am over here on the east coast and im tired.

CACalomino
04-05-2004, 12:11 AM
I thaught it would make it run richer or as we used to call it in the honda world LIMP MODE where the car will run but a touch richer to ensure the motor is protected if there is a problem could be wrong but I dont see why the car would lean its self out if it had a bad reading cause on a turbo car that could be a bad thing aka the big bang theory.


could be wrong mabe a teck coult chime in on this one


chris

Smogrunner
04-05-2004, 06:00 AM
I'm interested in this question too. I heard the second 02 sensor only "sensed." It does not change any engine parameters.

Blaze
04-05-2004, 12:09 PM
All:

Excellent information. Thanks for your input.

Now it's time to shop for test pipes. Anyone know which companies make test pipes with CARB numbers?

lol

Jason

Dr. Evo
04-05-2004, 12:24 PM
The second or downstream O2 has one purpose only: Catalyst efficiency. If it fails it will set a Check engine light with a Cat. efficiency code. It will not effect drivability at all. So the only problem with a cat delete is a possible CEL. And I don't believe that particular code will cause a limp-in mode, or anything. I will do some research and post again if any different.[/u]

drmosh
04-05-2004, 12:44 PM
My datalogger shows this when running test pipe:

If your downstream O2 senses an OBD2 Inefficiant Catalytic Converter, your ECU will do a Long Term Fuel Trim and richen the mix up.

Dr. Evo
04-05-2004, 03:30 PM
Interesting....
Like I said I am not porsitive. No one has been able to tell me for sure either way. If it does effect drivability, I guess the only way around it would be to put a O2 simulator in place of the rear O2. However, I called Mits techline and according to them, the code should not cause any problems. But who knows for sure? Apparently noone... :lol:

drmosh
04-05-2004, 03:55 PM
Interesting....
Like I said I am not porsitive. No one has been able to tell me for sure either way. If it does effect drivability, I guess the only way around it would be to put a O2 simulator in place of the rear O2. However, I called Mits techline and according to them, the code should not cause any problems. But who knows for sure? Apparently noone... :lol:

Well, I think the effect of the fuel mixture change is so small you can't really tell it when driving around the streets.

Dr. Evo
04-06-2004, 08:10 AM
That is a possibility. The only other thing I can think of as to why it would effect drivability is the assumation rule: The main reason that code would set would be a failed catalyst, not a failed sensor. The main reason a cat would fail is a misfire or a rich condition. So the computer will assume that the code is set due to a misfire or rich condition and start tweaking the mixture accordingly.