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Discussion Starter #1
I am new to this forum, but not new to cars and the car world. [burnout]

My car is a 2007 2.0L and the CEL came on and sure enough I got the P0420 code.

I have searched tons of threads on here about this code the variety of fixes that come with it. Based on the code description: "Catalyst System Efficiency Below Threshold (Bank 1)" it does not specify which sensor. Since its a I4 motor there is only one bank. Some of the other codes do call out bank 1, sensor 1 or 2 which would tell me more specifically what the issue is. My car has 3 O2 sensors actually. One of them is in the middle of the cat. I have worked on tons of cars over the years and have never seen this. So if it was one of the sensors I would guess it would be the downstream one.

This almost seems like I should go right to swapping out the cat doesn't it though? Or would it be something else? I have read some other threads mentioning the issue could be with spark plugs or the EGR.

Any help is much appreciated.
 

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the downstream o2 sensor measures catalyst efficiency. I'm not sure what your middle sensor does, but the first sensor is used to adjust fuel trims, and the last sensor watches the exhaust after the cat cleans it, and determines if its doing a good job or not. There is certainly a good chance your cat is not performing like it should and needs replaced. Much cheaper to try a new sensor first though.

or through the whole thing in the garbage and put a header in there and make some NOISE!
 
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Discussion Starter #3
Ha-ha yea I was looking at the cost of headers vs replacement cats and I may as well get a header...lol

Not really looking to do much performance mods to the car, but may have to start. =)
 

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Discussion Starter #4
I cleared the code and it came back after about 100 miles of driving. Seems like it wouldn't be that bad if it took that long to come back. I am going to replace the downstream O2 sensor and spark plugs while I am at it (need new ones anyway) Will report back if the problem goes or not away after O2 sensor is replaced.
 

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Sadly in my experience it's usually been the cat that needs replacing when these codes come up. Hopefully you don't have California emissions otherwise it'll get expensive very quickly. How many miles are on your car?
 

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Discussion Starter #6
Only 100k. Its hard for me to believe this is normal if the cat is the problem. I've owned two vehicles that I bought brand new and put over 200k miles on them and never had to replace an O2 sensor or cat on either of them.
 

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Discussion Starter #7
Finally got around to replacing the downstream O2 sensor. We will see if the code comes back this week or not.
 

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Hey Fixxxerask,

P0420 is a ball park code, no different than walking into a doctors office and telling them you have a cough or a fever. The context of the code and then searching for the root cause will save you a ton of money and headaches.

Do you have the Service Manual? It has a step by step workup for the P0420 code. In my case it was the perfect storm of issues that threw the code. Starting with the PCV hose and then spreading to other areas of the car. A bad O2 sensor is easy enough to diagnose with live data from OBD2 scanner. Local Autoparts store or a friend will have one available.

A replacement Cat will be the most expensive, so might want to consider working up your P0420 before swapping parts.

Did the code come back?
 

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Discussion Starter #9
I do not have the service manual. I heard of people using it though to diagnose these types of issues as well has many others. So far the code has not come back and I have driven about 100 miles. I have a 50 mile round trip to work everyday. If I make it through Friday then I'd consider it fixed. If not, then I will start to dive into it more with proper diagnosis and troubleshooting. O2 sensor was cheapest and easiest attempt to fix so I figured I'd try that first.

Then onto the next and final issue of the battery light randomly coming off and on...lol. I think I know what that is though and have already ordered parts for that.
 

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Battery light coming on and off can mean a few things, what did you order? And if it is the cat just get rid of it haha
 

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Cat delete is legal in a few spots in NA. If you plan on traveling with the car then... considered that option but ended up biting the bullet and getting a new cat.

This time around I'd likely do the work myself, but didn't have the experience or tools 2 years ago to tackle the job.
 

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It's not legal where I live, but they do not check. There's even a shop a mile away from the DMV that will do an exhaust without a CAT if you pay with cash lol
 

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Discussion Starter #14
I replaced the serpentine belt, idler pulley and tensioner last night. (just bought the kit that came with all three) The car only has 100k miles on it and the alternator looks fairly new and does not appear to be the original one. (I just bought this car about a month or so ago). I did some reading and found some people had issues with the belt and/or tensioner being the issue. The belt was making noise from time to time so that stuff needed replaced anyway. If this does not fix it though, I will check the grounding from the battery and check the plug/harness for corrosion at the alternator.

P.S. CEL still has not come back after O2 sensor replacement.
 

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Gently tug on each wire in that pigtail, mine looked intact put they would lose connection when I wasn't on the gas(in gear) which makes the engine move slightly. Tap of the gas and the light would be gone
 

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Discussion Starter #16 (Edited)
Well battery light came back on again after replacing the belt, tensioner and idler pulley. It never stays on for more than 10-15 seconds and sometimes and little as 2-3 seconds. It may only come on once or up to four times on my 25 mile commute to work. So strange. Guess I'll have to check the wires at the alternator like mentioned above and check the ground/return from the battery.

Also after 230 miles the stupid CEL came back on. I cannot believe it took that many miles. I have not scanned it yet, but I assume its the same code from before. I don't really want to replace the cat on this car. I only paid 2200 bucks for this car to use as a daily commuter to and from work. Nothing else. Gas mileage does not seem to be affected by this as I am getting 31-33 average total and I think that's pretty normal for these cars from what I have read. I might try to use a non-fouler to back it out of the stream to keep the code from coming back. I am not 100% if that will work, but I am thinking it might since that's what people do with headers installed.
 

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Discussion Starter #17
So I made a 200+ mile trip to Detroit yesterday and battery light came on a few times just in the first 10-15 minutes of my 3.5 hour trip. So weird. I am hopefully going to dive into that today and also put in a non-fouler for the O2 sensor.
 

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Discussion Starter #18
Installed non-fouler for O2 sensor. Drove about 200 miles and CEL is on again. This is getting ridiculous. If that is truly what is causing the issue then the non-fouler should have done the trick I would think. Unless its something completely different causing the issue.

Battery light is still intermittent. I went into the diagnostic mode and watched the battery voltage. its pretty much 13.9-14.2 while driving. The few times the light came on the voltage was at 14.2 so it appears the alternator is charging the battery, so something else is causing the light to come on. I hate these types of quirky electrical issues.

Not even sure what to do next on that. I guess I have a little more reassurance at least knowing the battery is getting charged and the car is not gonna die going down the freeway.
 

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Discussion Starter #19
Finally ordered an alternator to take care of the battery light. Will post back if that indeed clears it.

I am pretty much over troubleshooting and chasing down the O2 sensor code. I have two non-foulers in there right now and the code is still present. I can't imagine the cat is the problem here because the non-foulers should have done enough to make the ECU happy. I am going to take it to a local tuner and just have the code disabled. The car idles and drives perfectly fine and gets great gas mileage.
 

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This code does not indicate a bad O2 sensor. It indicates a bad catalytic converter. What the computer is doing is comparing the exhaust gasses before the catalytic converter and after. If the numbers are too close together it will throw a CEL, meaning the cat is not cleaning the exhaust gasses as well as it should. For you to be getting this code both O2 sensors would have to be working.

I get this code occasionally with my Mazdaspeed 3 which has an aftermarket downpipe with a higher flow catylitic converter. Because it's higher flow it doesn't clean as well and occasionally the before and after numbers are too similar and it throws this CEL.

I just clear it since I don't get it that often but there is a permanent fix for my car which should also work in your case if you just want to get rid of the CEL. It's probably not legal but if you put some washers between the second O2 sensor and where it connects to the exhaust it should throw off the reading enough for it to not throw a CEL.

Good luck.
 
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