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Old 06-06-2016, 09:59 PM   #1 (permalink)
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Default Code P0420 Help Needed

Hey, I've got a 2009 SX4 hatch 2.0L 4cyl engine, auto trans. 107k miles on it, and am having some issues pinpointing the issue causing the code. From what I gather, the P0420 code could be several things, from an O2 sensor, to the front CAT, to the MAF sensor, to just a simple vacuum leak. So, story timeI'll try to be brief but descriptive.)
I just bought this car a couple months ago, with no CEL on. I delved into the motor to see if it needed new spark plugs, and it did. While doing so I accidentally broke one of the plunger things that hold the air box/engine cover down to the valve cover. Maybe that helps hold it to the air filter housing/MAF sensor housing? I proceeded to replace the spark plugs, and when doing so let a small amount of oil into the cylinder that had leaked in from underneath the spark plug well seals. Going back together, I also replaced the air intake filter. Upon startup it burned off the oil that had fallen into the cylinder. Soon after, the CEL turned on. I had the code read by O'riellys, and they quoted me P0420, bank 1 too lean. I figured it was incidental due to the burnt oil and left it for a couple of days and the code disappeared. It came back, this time the code for a bad MAF sensor reading(I forget the code #). Unhooked the battery to get it to go away to see if it comes back, and yes about a week later it came back P0420. I tried cleaning the MAF sensor, no luck. Right now I'm thinking it's either a bad O2 sensor or clogged CAT.
My question is; how probable is it that it's either the O2 sensor vs the front CAT (or something else entirely) and what are some indicators that would lead me in the right direction? I know how to test a CAT using an infared thermometer, but am unsure what temps I'd be looking for, and I have no idea how to test an O2 sensor. I may have access to a scan tool that could give me live data but am unsure what readings I would need. I'll be doing all the work myself so I'd appreciate all the details and instructions anyone could give me. Lay it on me!
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Old 06-07-2016, 11:37 AM   #2 (permalink)
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When I worked in the test & evaluation section of CAT's research center, we tested O2 Sensors. We found out that a brand new O2 Sensor will run the engine a little on the lean side, but as they age they drift more and more toward the Rich side. (wasting fuel).
Usually by 60k miles, or so, they have drifted off center to the point that they should be changed, to increase performance and MPG.

I changed the O2 sensors on my Chevy X11 at about 65k miles and increased both performance and MPG by over 10%.

So that might be a good place for you to start.

Good Luck!
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Old 06-07-2016, 06:19 PM   #3 (permalink)
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I remember reading that in another forum, but with my case the O2 sensor failure or whatever says it's causing the car to run lean, not rich as you said they naturally degrade to. Plus I'd like to be as sure as I possibly can before I replace the O2 sensor, as getting a good replacement is going to run me nearly $200... a decent chunk of change for me right now. I heard on another thread that there may be a post to a forum on here that discusses cheap alternatives to factory replacement O2 sensors, but I haven't been able to find that thread. Anybody know where it happens to be? Thank you Shadow for your input! If anyone else has some insight or experience with this issue I'm definitely looking for all the info I can get before I do something.
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Old 06-07-2016, 06:26 PM   #4 (permalink)
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Granted, I've not replaced an O2 sensor in several decades, but, I'm pretty sure you can find them a lot cheaper than $200.
First check your local parts houses and then check "Rock Auto".

The last one I changed, years ago now, was just $25 from a local parts house.

Good Luck!
Shadow
PS: You probably won't get one from your local Suzuki dealer.
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Old 06-07-2016, 09:42 PM   #5 (permalink)
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Well I searched google shopping which linked me to ebay for an upstream O2 sensor for my vehicle and was surprised to find options for nearly $30. Looks like I'm going to give that a shot!
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Old 06-08-2016, 05:10 AM   #6 (permalink)
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Quote:
You probably won't get one from your local Suzuki dealer.
There are an abundance of Dealers that maintain part inventories.

Just a few of them are listed here...
https://www.suzuki-forums.com/2g-1999...technical.html
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Old 06-08-2016, 10:11 AM   #7 (permalink)
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It might be beneficial if you conduct a proper Live Data Scan, rather than guess, or use a code reader..

Yes O2's are maintenance items, but throwing in unnecessarily is a waste and will not fix a contributing issue...

Regarding the air box, make sure you do not have an illegal air issue...

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Old 06-10-2016, 06:30 PM   #8 (permalink)
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I wouldn't necessarily call it 'guessing' if two error codes read "bad o2 sensor" and there aren't any other causes of running lean that I can find. I've checked everything for vacuum leaks that I can. Cleaned out the fuel fill tube, and the gas cap, checked to make sure the fuel pump was working, as to rule out low fuel volume in the intake mixture. I'd rather call it 'hoping with relevant evidence'.
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Old 06-10-2016, 10:08 PM   #9 (permalink)
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Maybe I'm missing something, but the Suzuki manual I'm looking at says a P0420 is "Catalytic System Efficiency Below Threshold" - nothing on bad O2 sensors, or running lean.
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Old 06-13-2016, 11:07 AM   #10 (permalink)
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All the code readings say for p4020 I've gotten say bank 1 too lean. The all of the direction I've received pointed to the o2 sensor. I feel like I'm in the right place now because now that I've replaced the sensor with a cheap eBay one it gives me a different error code, and not the p4020. The new error code is p2237. O2 sensor stuck open, or a short in the wiring? I'm going to see if the eBay seller will exchange the sensor for a new one so I can determine if it's just a bad part. The car is driving a bit weird, as if the sensor can't get a good reading and it's switching between lean and rich and slightly surging under acceleration. Anyone have insight on this movement in the plot?
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