I have recently received my STi stage one from Vishnu. I put the exhaust system (but not the XEDE) on this past weekend and had a question.

Why is the O2 sensor located downstream of the catalytic? This is not the stock location and I have been doing forum searches (and a few phone calls) but can not find an informed answer.

Dosen't subaru's ecu "look" at the 02 sensor for a/f information. Placing it downstream of the cat my A/F meter reads lean all the time (hydrocarbons being scrubbed obviously). Now I'm wondering what my ecu sees. Is it seeing lean too? Is it adjusting for this? I would love some information from Shiv if possible on this one.

Otherwise, it sounds GREAT! And as badly as I want to hook up the XEDE to enjoy the benefits, I am still going to wait and see how quickly the harness for the STi will be ready.......I am reluctant to "hack the harness" as it were.

Thanks !

 

Well, so far the story gets worse...... Went for a long hard drive around the mountains here (yeah baby) and the CEL came on with the cruise flashing. Pulled over, reset the ecu and started up again. 20miles later, same thing. No safe mode or anything, just a CEL. drove easy home and now I'm wondering who in town has an obdII reader.....

the only thing I've changed on the car was the exhaust system. Never had a CEL until today. Can't help but think that this is a result of the O2 sensor behind the cat. I REALLY don't want to hurt my STi. Wish there was more information about this....

Shiv... anyone... could use a hand.

Thanks
vaj

 

The rear O2 isn't used by the ECU for tuning (that's what the front one in the manifold is for), but is used for emissions purposes. The CEL you are getting is most likely the cat inefficiency CEL. This is the result of removing the downpipe cat. Whether you had the O2 sensor in front or behind the last cat, you would still most likely get this CEL. When I had my WRX, I had removed all of the cats except the last one and I got the CEL. You have to understand that catalytic converters work off of heat. The cat in the downpipe runs most efficiently because it is right behind the turbo.

I can see why Vishnu would put the O2 sensor behind the cat. They are hoping the hiflow cat will do it's job and the ECU will think the exhaust is being scrubbed properly. If it were in front of the cat, with no cats between it and the engine, you would get an instant CEL.

There is a very easy fix for this problem. It involves fooling the ECU by installing a resistor inline of the O2 sensor wiring. Do a search for CEL fix and it should come up. Be warned you will probably find two types of CEL fixes. One is for the WRX uppipe EGT and one is for the WRX&STi rear O2. Make sure you get the right one.

Also, I am just guessing what your CEL is for. You should get the code pulled to be certain. The rear O2 sensor is P0420 if I recall correctly.

 

I went ahead and bought an OBD2 scanner (<$150). Now I can look up any codes that flash. Now if someone needs a code pulled, then plug it in, and there's the code. I got mine from SCHUCK's, or autozone type store.

I actually, tried 2 of them. The first one wouldn't work. I did the testing at the store and returned it. The second one worked and away I went.

 

I think there are 3 different protocols for OBDII readers. Most Japanese, Euro, and Chrysler cars are ISO. Ford is PWM. GM is VPW.

Some readers only do one of the three, some will do all of them.

Also, Delta Dash, which is a wonderful logging tool, will read fault codes for our subies as well.

 

OK. Looks like I'm on the right track here. Speaking with a few folks on the phone (Dwight from GMK Performance et al) Seems like cat inefficiency is the problem and probably the code I'll get at the shop today. I realized that the o2 sensor behind the main cat is seeing little heat due to the hiflow nature of the turbo back. This o2 sensor is metered (I guess thats not quite the right sensor to meter huh) and is reading jenny craig (LEEEEAN) all the time.

However. I've been running this car without the downpipe cat (had it replaced with Perrin's catless DP early in car''s life) for many months with no CELS. The o2 sensor is the autometer type designed to work with Subaru's and their A/F meter. Pre-vishnu, the sensor was located in the stock Subaru location just ahead of the cat... Perhaps it just got more heat there. Even though I know now that this is NOT the o2 sensor used by the ecu for a/f and Engine management the read out on my gauge (fine) and the lack of CEL's indicated that the ecu was not disturbed by this.

I guess I'll search for the CEL fix and let you all know how it turned out today!

Thanks for the helps yall.....