Okay, so I brought my '03 evo 8 to Mitsubishi to find out how much it would cost to get a new exhaust system. My evo has a CEL on because it is missing an 02 sensor pre catalytic convertor. I know there are supposed to be 2 cats on this car, but I only have one on the mid pipe. Can anyone tell me exactly how the stock exhaust system from the turbo back is? I see that I have an 02 sensor right on the exhaust header and I also have one after the back CAT but I only have one CAT instead of 2. What pieces are missing? I'm trying to figure out what else I could do to make the CEL go off (ex: test pipe or something to fool the computer into thinking there is a CAT where there isn't.) They said that the pre-CAT sensor regulates the air/fuel mixture and that's why I'm running rich: because there is no 1st CAT or sensor. I just want to figure out how a stock exhaust system is set up on an evo 8. Thanks.

 

There should be an O2 sensor on the O2 housing. You definitely need this. That sensor is pre cat. There is another sensor on the cat/test pipe.

 

There is only one cat on the car and it is in the test pipe location like mentioned.

 

Lift the hood and look at the manifold, you will see the first 02 sensor plugged in the left side as you look down at the exhaust--as mentioned above-, that 02 regulates the mixture---car wont run without it. The second one you found is down by the cat and sets off the cell if the cat doesnt work.. Again as mentioned there is only 1 cat--there never has been 2. Pull the code and figure out exactly what the issue is--, bad 02 or bad cat, or no cat bacause someone thought they could run a test pipe of HFC instead. You have a complete system, you just need to pull the code and work the issue.

 

i didnt know we were dual exhaust lol. 1 pre cat and one after cat.

 

I'm just going by what the dealer at mitsubishi service told me. I have both sensors on my car. The code is a P0172 (System too rich bank 1). In his report of the car he labeled "missing o2 sensor pre-cat" I can see one plugged in on the bottom of the exhaust headers and the other on the mid pipe. It has an aftermarket cat on it to pass NY state inspection, but he said it's missing another. I don't know how that could be. He clearly mentioned 2 cats. I don't know what sense that would make on a single exhaust car as mentioned in one of the previous snide comments. Anyway, the exhaust is still messed up and causing a rich running condition. Don't know what else to do.

 

One o2 sensor is located in the o2 housing just after the turbo and the other is in the test pipe.

 

I can clearly see the two sensors, but how can mitsubishi say it's missing one if it's required to run? I drive the car everyday, I know it runs. Could it be that the first o2 sensor is defective causing the code by making it run rich. The guy I originally bought it from said he got that code because of the exhaust system. It's just a huge pipe with a magnaflow cat welded in and it goes back to the muffler. He removed a lot of the stock pieces but I know there are 2 sensors I have on it.

 

Yes, your info was the most helpful, now you say "someone thought they could run a test pipe of HFC instead" The car does have a high flow magnaflow cat on it to pass NYS inspection. As I mentioned after your post, the car is running a P0172 (System too rich). Since there are both sensors on there, what on the exhaust could be giving me this code and where should I check next? Can a sensor be defective? Or can the HFC be giving me this code?(it's new) It gave me this code before the cat was even installed though-keep that in mind. Thanks for your input.

 

reset the code and see if it triggers again. the HFC could not be sufficient enough and u might need a cel fix mod. o2 sensor extender
it cant pass emissions with that code lit up?

or it could just be the primary o2 sensor (the one under the hood) being faulty since u "said" ur running rich.

 

Thanks, I have been resetting this on and off for 6 months now, so I know it keeps triggering it. The car won't pass inspection at all with the light on so I just pray it stays off for the inspection when it's due (it's random when it triggers back). Could you explain about a cel fix mod? This can also mean a faulty o2 sensor? That's not a seperate code? Thanks for your input. Really appreciated.

 

You can try a mechanical trick with the spark plug non fouler that pulls the O2 sensor away a bit or you'll have to go in your rom on the ecu with ecuflash and run the O2 heater patch fix and disable secondary O2. Contact a tuner if you are not familiar with this.

 

You're in NY, you should call STM and see if they can help you.

 

I very much appreciate the recommendation but STM is in Rochester NY which is a 7 hour drive from where I am in NY. Looks to be a bit too far for me lol, but thank you all the same.

 

A little bit of info here that might help you out...

Most modern day cars have oxygen sensors at 2 different points along the exhaust flow path. The first oxygen sensor, usually very close to the engine, is used for fuel mixture trimming, and is also used to detect overly lean or rich fault conditions. This is the sensor that detects P0172. The downstream O2 sensor, which is after the catalytic converter, is used to test catalytic converter efficiency - so it is used for emissions compliance primarily. Also, FYI - "bank 1" refers to the side of the engine that has cylinder 1 on it - this nomenclature is only relevant for V configuration engines like a V6, where there are 2 sides (or "banks"), and unique sensors for each one.

The Evo 8 and 9 is a fairly simple setup. These models have ONE catalytic converter underneath the car. There is 1 O2 sensor right after the turbo outlet near the exhaust header, and the second after (or actually inside) the catalytic converter. Some cars do have 2 or more catalytic converters and some V6 and V8 cars even have 4. But the Evo 8 definitely has 1. The issue you are having is being detected by the first O2 sensor up near the cylinder head. BTW - the fact that a dealer is telling you your car should have two cats is a bit of a red flag to me.

There are several conditions that could cause P0172. In fact, it is a fairly common code, so many of the internet suggestions could be applicable. An exhaust leak could cause it, as could a fueling issue, a bad O2 sensor, or a MAF sensor problem, among others. The problem is not always (although it could be) an exhaust issue.

Unfortunately, there are a few things that will need to be checked out to fix this code, hopefully it is not too complicated or troublesome to fix.

Edit: Although perhaps implied, let me clarify that I do not think your aftermarket cat is likely to be causing this issue, although anything is possible I suppose. The exhaust leak would more likely be near the first O2 sensor, and probably upstream of it to be causing this issue, if in fact, an exhaust leak is the culprit.

Good luck.