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Discussion Starter #1
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 time:(I'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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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!
The Shadow :cool:
 

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Discussion Starter #3
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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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 :cool:
PS: You probably won't get one from your local Suzuki dealer. :rolleyes:
 

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Discussion Starter #5
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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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...

... Philip
 

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Discussion Starter #8
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'. :cool:
 

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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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Discussion Starter #10
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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Discussion Starter #12
SOLVED- for the most part

Fordem was definitely right in pointing out to me that the code P0420 has to do with the catalyst system, not the O2 sensors. I was misdirected by the employees of Autozone and Oreilly's to believe that because the code read "bank 1" that it meant the first sensor in the stream (the upstream sensor). Bank 1 only means that it was the first exhaust system on the engine (and since its a 4cylinder it only has 1 exhaust system anyway). So if you had a V6 or a V8 there would be TWO banks, one on either side of the engine. Basically, don't let auto parts stores sales associates lead you astray.

I replaced the catalytic converter with the cheapest thing i could find on ebay and sure enough, the code hasn't come back. That doesn't mean there isn't a pre-existing issue to cause premature failure of the cat. I'm looking into that as well. Riiiiiight after I replace the radiator. I dropped a nut into the fan shroud while replacing the cat that must have gotten kicked up into the fan and shot into the radiator. Wish me luck!
 

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The O2 sensors are the "sniffers" that sample the exhaust gasses and trigger the coding. ;)

That said, a bad sensor can provide erroneous code setting which MAY have indicated a faulty Cat, but the Cat itself could have been in spec. :rolleyes:

Time will tell.
 

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Here's how it works...

The OBDII readers will specify B1S1 (Bank 1, Sensor 1) and B1S2 (Bank 1, Sensor 2) for an "inline" engine which only has one bank of cylinders, and if it's a V engine with two banks of cylinders, you'll also see, B2S1 (Bank 2, Sensor 1) and B2S2 (Bank 2, Sensor 2)

Sensor 1 is known as the "upstream" sensor, because it is located in the exhaust before the catalytic converter, and sensor 2 is known as the "downstream" sensor, because it is located after the converter.

Sensor 1 is the one that controls fuelling - it monitors unburned oxygen in the exhaust before the catalytic converter and adjusts fuelling appropriately, a failed sensor 1 can cause an incorrect mixture, but it is more likely to be rich rather than lean.

Sensor 2 has nothing to do with fuelling - its purpose is to monitor catalytic converter efficiency, which it does by monitoring unburned oxygen in the exhaust after the catalytic converter - if the catalytic converter is doing it's job (burning unburned hydrocarbons or converting CO to CO2), there should be less oxygen in the exhaust after the catalytic converter, so, if/when the output of sensor 2 closely matches sensor 1, the ECU flags the converter as failed with a P0420 error code.
 
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