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So I just read what is going on with your CEL p0420 code. im in the same spot as you .
Hey everyone I just made an account here because i am at my wits end with this stupid car issue. My check engine light has been on since about 70,000. I have 110,000miles now. The code comes up p0420 which is the Catalyst System Efficiency Below Threshold Bank 1. I did some searching around and found a few people claiming its either the rear O2 sensor, the front O2 sensor (or A/F sensor??), or the catalytic converter. everything i read from people with my car turned out to be the cat. I took it to the shop yesterday, they did a diag. for free and it is a bad cat. Last year i did the whole disconnect my batt. and drive it for 50 miles the cel didnt come back on and all my sensors reset and i passed smog. This year, not so much. I did the same thing and im at about 100 miles the CEL still hasnt come on, but the catalyst and evap sensor still havent turned back on. so no pass on the smog. I just need one more to reset and ill be ok.but usually they seem to both reset at the same time and CEL come on. I have to register my car in like three days and i just dont know what to do. I dont really have the money now to get a whole new cat put in $720.00 the guy told me installation and all it's (not under warrenty anymore). ive heard of this anti fouler thing. I have no idea what it is, where to get it, and how to do it, and if it would even help in my case to pass smog. . the thing thats messing me up is that those two sensors that need to be complete when i smog will not reset or whatever this time. first time i unhooked batt, they all reset after 40miles, next time they reset at 70miles CEL came on too fast for me to even smog it, not its sitting in my drivway 100miles since i reset the CEL took it for smog and they were not complete. im so frustrated. if anyone has any suggestions for me i would greatly appreciate it. thank you

so what cat is it the one on the header or the stock pipe? or u could buy a scangauge and clear all check codes...where u located at

The P0420 CEL is common to people who run aftermarket headers or turbo their cars. Basically the CEL is thrown when the readings at the secondary O2 is the same at the first prior to the CAT. At your mileage with no mods it's most likely a failed CAT.

Here's a detailed explanation of the fix
http://www.scionlife.com/forums/showthread.php?t=45083

and here's a visual (skip down to after the header was installed and you'll see some thing that looks like a long aluminum spacer added between the O2 sensor and the s-pipe)
http://www.scionlife.com/forums/showthread.php?t=22180

in short, this attempts to restore a variance in readings between the secondary and primary O2 sensors. I only say "attempts" because the reported results are varied from unsuccessful to the problem being permanently solved. But for the relative pennies this mod costs, it's worth a shot.

a shot of the part needed:
http://www.scionlife.com/forums/showpost.php?p=2500348&postcount=5

so what cat is it the one on the header or the stock pipe? or u could buy a scangauge and clear all check codes...where u located at the primary cat responsible for the CEL is the one integrated into the stock header. I would also recommend the OP buy a scanner but if he's just clearing codes, pulling the battery is doing the same thing.




EDIT: plan B if time allows - get a used stock header second hand from a junkyard or part-out. Fixing this shouldn't cost you $720. This is all a repair shop would do anyway.

ooook, i see now what ur saying about looking AFTER the headers are installed. now that anti fouler is that going to be installed to the o2 sensor i can see right when i pop my hood? or is it the one on the bottom after it goes through my cat??

so the more im reading ur instructions the more im understanding them. so it is the one on the bottom after my cat. im going to try and attemp to do it today. Then hopefully when i drive it enough and all my sensors will be good,, no cel, and i can smog it.

Here's a picture of the stock header removed from the car - as you can see the CAT is integrated just below the primary O2. Looking at the lower flange and mounting brackets I don't imagine a shop doing anything other than replacing the whole unit.

http://www.scionlife.com/forums/attachment.php?attachmentid=2502&d=1278513597

here's some ebay listings that should give you a rough idea of what they go for used:

http://shop.ebay.com/i.html?_nkw=scion+tc+oem+header&_sacat=0&_dmpt=Motors_Car_Truck_Parts_Accessories&_odkw=scion+tc+oem+header*&_osacat=0&bkBtn=&_trksid=p3286.m270.l1313




ooook, i see now what ur saying about looking AFTER the headers are installed. now that anti fouler is that going to be installed to the o2 sensor i can see right when i pop my hood? or is it the one on the bottom after it goes through my cat??
sadly it's the one under the car past the header :(

I have a stock one for you if you need a new one. only 9K miles on it. PM me if your intreted