I searched and couldn't find a difinative answer on this.

Does the stock ECU use the 1st or 2nd O2 sensor to measure A/F ratio and adjust accordingly? I would assume the 1st sensor (the one in the O2 housing) but I've been told that it also uses the 2nd sensor (the one after the cat) for some adjustments also. I was under the impression that the second sensor was just there to make sure the cat was working.

Anybody care to clue a newbie in on this?

Thanks,

Corey

 

1st but it is really only good for a small afr range, that is why it is normally called a narrow band sensor. The one after the cat in some cars does make small adjustments but i dont think in the evo it does not. It just makes sure the cat has lit off and is burning the garbage

 

To add on Spaceball 1's comment, the front sensor is really only good for finding stoichiometric (roughly 14.7:1). The ECU can determine if it is richer than that point or leaner than that point, but not really how much (at least with any precision).

Closed loop, the ECU uses this sensor to try to maintain stoichiometric, typically by sea sawing back and forth between just rich and just lean. It does this solely for emissions reasons. Better power and cooler operating temps can be found rich of this point and better fuel economy and cooler operating temps can be found lean of this point. However, this is the spot that cats work at best efficiency, so we run there much of the time.

The rear O2 sensor is intended to monitor cat efficiency. UEGO sensors require hydrocarbons to properly measure rich of stoichiometric and cats remove hydrocarbons, so there is little meaninful information that the sensor can provide the ECU other than cat operation. It is true that removing this sensor can lead to things like rich idles on some vehicles (though I do not believe that Evos are among them). However, the reason is normally one of two things. Either the removal of the sensor and heater circuit causes a ground shift to the front sensor, or the ECU responds to the removal of the sensor by shifting rich to protect the cat. If fueling is shifted rich, then the cat cannot collect any O2. Since there is no O2, the cat also cannot spectacularly overheat.

-jjf

 

zt-2 ALL THE WAY zeintronix

 

+1