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Discussion Starter #1 (Edited)
So my 2003 Elantra has been getting horrible gas mileage, running super rich. I had a constant P0420 catalytic emissions code and one misfire reading. I'd clear it and it'd go away for about an hour and come back. I feared I would have to replace my whole exhaust manifold/cat part until I read this forum about maybe having a faulty downstream O2 sensor. Sure enough I replaced it this weekend and the code is no more, my car feels brand new except for my soft 10 year old springs haha. Now I can get smogged and get registered for 2014.

I just wanted to thank you guys for the very helpful resources, I hope anyone having similar problems considers changing the sensor first before the cat. Its surprisingly not that difficult with the right tools and PB blaster. You can do this job with ramps, that's all I have at the moment. There is enough clearance and leverage if you don't have a jack with stands available.

2003 Elantra O2 - Bosch Part Number: 13461
$40 on Amazon, I didn't want to wait for shipping to do it this weekend. It is $70 at Autozone
3/8" Offset Oxygen sensor socket - $6
I highly recommend this tool from Harbor Freight over the tall socket, this paired with the new 17" 3/8th drive breaker bar I got came off like butter in one try.

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Thanks for the info and prices. Our mileage suddenly took a nose dive from 25 MPG in town to 18 MPG over the last month. But I haven't gotten any codes. I'll try this once it warms up.
 

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Downstream oxygen sensor is pretty easy to replace. The upstream oxygen sensor is a little difficult to remove because of its angled location under the exhaust manifold. So did your mpg improve after the change? How did you determine that it was downstream O2 sensor, not upstream?
In my case the P0420 code disappeared by pouring a can of Seafoam or on its own for couple of months. So if the error code just disappeared, it may be too soon to declare it's over.
 

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Discussion Starter #5
Downstream oxygen sensor is pretty easy to replace. The upstream oxygen sensor is a little difficult to remove because of its angled location under the exhaust manifold. So did your mpg improve after the change? How did you determine that it was downstream O2 sensor, not upstream?
In my case the P0420 code disappeared by pouring a can of Seafoam or on its own for couple of months. So if the error code just disappeared, it may be too soon to declare it's over.
My mpg has improved a bit, but I still have to deal with a lot of city miles and I have a lead foot; average around 23/27mpg. I determined it was downstream at a shop with the high tech OB2 scanner and it showed the 2nd sensor bouncing all over the place while the upstream was pretty steady. And the fact that the downstream was so much cheaper to replace lol, for my 2003 GLS with the California super low emissions vehicle the upstream is like $125 bucks so I figure try to fix the cheaper one first.
 

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Only way to tell is by the Check Engine Light. Which then, if you use a code scanner it will throw the same code for either one. It's an OBDII limitation. It's a universal system that car manufacturers agreed upon using.

Some manufacturers add additional things like ABS, AirBags, etc... and then only their dealer tool will read those. I haven't seen a scanner that can distinguish between the two, even the most expensive from parts stores don't. Might if you have a shop/dealer scanner.

You can either do trial and error which could be expensive, because can't return the part if it's used.

Or you can try to read the resistance on each sensor to see if they are in proper range.
I don't know the resistance for each. I looked in my shop manual and it wasn't listed anywhere.
 

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In my case, my '03 gls with 307, 000 miles on it has average mpg 30-32. When there is P0420 code, a little less, but I don't feel any lack of power or shaking. Last week I pull out spark plugs. They were not bad, but the gap became bigger from original setting and some carbon deposit. I cleaned them and readjusted. I think I have to replace them within a few months.
 

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For several days, my P0420 code didn't go away even after I poured a can of Seafoam into the gas tank several days ago. Usually the error code disappeared in a day or two. My converter may be severely clogged. Before attempting to replace it or dismantling it to physically clean it up with detergent, I poured a can of "CataClean" into gas tank today. Has anyone used CataClean? It claims that it can clean catalytic converter and O2 sensor. It is recommended that I should use CataClean four times a year. It's quite expensive, though. One pint can costs $25 + tax. I bought it at Autozone. I will drive about a hundred miles tomorrow and I will see whether it will help or just waste of money.
I was afraid of using lacquer thinner as Scott Kilmer recommended.
 

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Idk whats going on I just replaced my bank 1 sensor 2 (downstream sensor) I got a ntk 02 sensor from rockauto.com so It can pass emissions. So I put it on & drove it to get ready & got it ready took it to my mechanic to see if its ready, & its ready Which it does pass emission & everything because my car is my daily driver & I was coming home & so the CEL came on for the bank 1 sensor 2 which I just replace does anyone know what could be the problem should I clean the sensor connector any suggestions.?? Thankz

KDM_STANCE_XD2
 

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Even though the scanner says sensor 2 or sensor 1, it could be caused by the problem with your clogged catalytic converter. Try CataClean and let other members know whether it works or not.
 

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So it doesnt matter what sensor shows up it could be a catalytic converter being clogged & making the sensor to fail am I right if CataClean that expensive it should be doing something right

KDM_STANCE_XD2
 

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Ill probably be getting it tomorrow I drive my car daily once that pours in should I erase the CEL n just drive it.?

KDM_STANCE_XD2
 

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1
How do I maximise the results of Cataclean?
Follow these simple instructions: 1. Make certain that you have ¼ of a tank of fuel in your tank (3-4 gallons) 2. Pour entire contents (475ml) of Cataclean into the fuel tank. 3. Drive vehicle for 10 -15 miles

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What are some of the things that I should look for that would indicate that my vehicle is not operating at its best?
Your "check engine" or "service engine" light has come on. Your MPG (miles per gallon) has declined. Your engine is sluggish. You may fail your "emissions test".

3
How often do I use Cataclean?
It is recommended that one bottle of Cataclean (16.7 fluid ounces) is introduced into your fuel tank, once every three months. If your vehicle is subjected to a lot of stop and start or minimal highway miles, you may consider adding every 60 days.

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Is Cataclean safe for my car?
Yes. Cataclean has been extensively tested over the last 10 years by Industry Experts and is safe for use in both diesel and petrol engines and dual-fuel engines.

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Will Cataclean really turn off my check engine light?
Check engine lights are often triggered due to carbon build up on the oxygen sensors and within the catalytic converter or mechanical failure of one or both of these components. Your mechanic will refer to this as a PO420 code. In the case of mechanical failure of the oxygen sensors or disintegration of the substrate within the catalytic converter, nothing, not even Cataclean will address this…you must replace the part. In the case of carbon build up and not mechanical failure, Cataclean has been proven to remove carbon build up which has resulted in turning the check engine light off.

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"CataClean". The employee there didn't even know it until I found it on the shelves. I drove 50 miles this morning after pouring a bottle of CataClean last evening. The CEL has not yet gone, but my car runs super fast and I feel it has lots power boost. I will keep you updated later this evening after I come back home.

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They didnt wth lol & well im glad it feels like that bro & alright keep me updated I hope it works out good tho

KDM_STANCE_XD2
 
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