Mercenary3, thanks so much for your input. Really appreciated. I have tried many things to clear the code and have been unsuccessful. I did all the things mechanics suggested like boost leak tests, maf sensors, intercooler hoses, etc. My main concern now is the sensors as you have mentioned. As you said, you don't think the CAT is the culprit, which is good. As for an exhaust leak, it is also very likely since the exhaust was changed. I'm still shocked at what the dealer told me though, I'm not a mechanic but I'm no idiot as well but that day I was arguing with them so much over the prices, I think they were just trying to get me out of there because I was making a scene :/ I'm thinking of checking out the sensors and seeing if those are contributing to the problem. Your information was phenomenally detailed and I appreciate your help. I will keep all of your words in mind as I continue to investigate the problem. Many thanks!

 

People like Mercenary3 is amazing, thank's for your time to make really good and clear explanation.

If the problem is only the check on. A cheap (and not so good) solution, is to remove the check engine light bulb on the cluster, maybe 30 mins to do that. That happened to my with my VIII, I saw the codes with the evoscan but no lights, when I installed a rexped Evolution logo I saw no bulb.

And Maybe you can try with a minicat on the test pipe, the rear o2 sensor will receive a different reading than the front.

But that is only a "mask" not a complete solution.

Good luck

 

Check and clean a few things:
MAF
Check all vacuum hoses
Check for an exhaust leak.
Some one else suggested the spark plug extender, for the rear O2. definitely do this, its cheap and a good potential fix.
Consider getting a used down pipe with the correct flange and stock cat that will bolt in instead of the welded HFC that you have to remove that issue.
Lastly, you may need to replace the frt 02..

These are just suggestions from least expensive to most. At some point it will be fixed.

Yes a HFC can and in most all cases does trip the code.

BTW---here is the mitsu P0172 diag list.

Possible causes
- Faulty front heated oxygen sensor
- Ignition misfiring
- Faulty fuel injectors
- Exhaust gas leaks
- Incorrect fuel pressure
- Faulty Mass Air Flow (MAF) sensor Help with this
When is the code detected?
- Fuel injection system does not operate properly.
- The amount of mixture ratio compensation is too large. (The mixture ratio is too rich.)
Symptoms
- Engine Light ON (or Service Engine Soon Warning Light)
- Excessive Fuel Consumption
P0172 MITSUBISHI Description
With the Air/Fuel Mixture Ratio Self-Learning Control, the actual mixture ratio can be brought closely to the theoretical mixture ratio based on the mixture ratio feedback signal from the heated oxygen sensors 1. The Engine Control Module (ECM) calculates the necessary compensation to correct the offset between the actual and the theoretical ratios.
In case the amount of the compensation value is extremely large (The actual mixture ratio is too lean.), the ECM judges the condition as the fuel injection system malfunction and light up the MIL


Read more: http://helpforcars.net/obd_codes/p01...#ixzz2toHrQVAu

 

Thanks again everyone, really appreciated. I did do some of the steps in order to clear the code in the past (unsuccessfully). I have changed the MAF sensor, replaced the vacuum hoses. I know the previous owner had the ECM tuned and it runs rough in addition to the exhaust mods. The previous owner mentioned the code being shown because he removed the cat. I had a cat installed only to pass the inspection and the code is still being displayed. The pipe has a larger diameter than the stock pipe so I had the aftermarket cat welded in. I'm thinking if I could find a decent tuner, I would have them look at it and maybe de-tune the ECM. My original goal was to return the car to almost stock but I'm still trying to figure out this main problem. My mechanic had no idea how to fix it and Mitsubishi quoted me $2200 to replace the exhaust from the turbo back. They also were unsure because of the tune on the car. They attributed it's rough idle to the computer tune. I'm trying to get this fixed but there's so many different factors contributing to it. Anyway, I'll continue to look in to the problem, I really appreciate everyone's input though. Any info is greatly appreciated.

**one other thing: do you think it's worth changing the front o2 sensor for the heck of it? I was thinking is this worth it just to do a quick replace and see if it solves the problem??

 

Not a bad idea to change out, if you are able to, and willing to pay (I guess considering the quote from Mitsu, not a bad option to try first!). Those sensors can be a little delicate, especially the wiring. If the previous owner was messing around, you can cause internal damage to the sensor without it being obvious on the outside. I've done exactly that on my own downstream O2 without knowing it.

 

Thanks Mercenary3,
I'm going to look into some o2's when the weather warms up. I figured since it's a rather easy DIY, might want to try it myself. I've been unsuccessful with just about everything else, might as well add another to the list lol. But I will keep all of your advice in mind while doing it.