CEL code P0420

when the check engine light went on about a week after i bought the car, i took it back to the dealership (it had a one month/1000 mile warranty) and they replaced my fuel pressure regulator (code P1298). $214.55 worth of parts and labor FREE wooo, but on the invoice it even says "codes may come back on at future date for cat." so the light came back on later, then it went off, then it came back on, and its been on since last thursday after i got gas. so now what am i going to do with the P0420 code? Chances are, short of replacing every O2 sensor and the cat, i wont be able to find the right thing to replace. ive done some research on the internet and came to the conclusion: the internet is worthless. some people even said their egr valve was the cause. what should i do? theres no way im spending a grand at the dealership for the cat, and the one obd tech i talked to said the vehicle couldnt pass emissions with an aftermarket cat/downpipe. ive never had to take an emissions test, either. id be ok with the light staying on, but on the last 14 gallons of gas i used, i only 299.8 miles on it--about 19mpg. my drive to work is a straight 4 mile shot in a 40 mph zone. my first 2 tanks of gas i got well over 320 miles on each. i need help!

Well, first things first.
When the CEL came on, was it solid or blinking?
If it's solid, that means the ECU detected a problem but rectified it and the code was stored. If that was the case, the CEL will go off. OR what you can do is remove the back up fuse under the hood for about 30 seconds and that will reset the CEL. But that's only if the CEL is solid and not blinking.
If the CEL is blinking, the ECU detected a problem and was NOT able to rectify it. What you have to do is pull the code to see what's the problem.
There are two ways of doing this.

1) take it to autozone and have them pull it for free.

2) Click here and use the following steps to pull the code yourself then see what the code means.
Right now the code P0420 means Catalyst System Efficiency is below Threshold.
Personally, I'd take back to the dealer. Especially if it was stated on the previous repair envoice. The dealership is responsible for making sure the car passes emissions inpsection.
As for the poor MPG's, make sure the obvious is taken care of. proper tire inflation, fuel/air system clean, plugs/wires ok etc....
I'm sure the dealer will try and bring one of those up as a reason for the poor gas mileage.
Good luck and keep us informed.

yea, i have the catalytic converter one on and off every now and then, haha.

currently: ON
yesterday: OFF

haha

well, hopped in the car today to run some errands, and the light was off (again). hmmmm...strange, considering the fact that the car was running like a diesel (again).

i forgot to mention that it was a "bank 1" sensor, so i think its the upstream one. the light has never blinked, its always stayed on and gone off when it feels like it. using the guide on this forum, the code i got doesnt exist. i think it was a 67, and theres only a 66 or the other way around.

as for autozones, the nearest one is a half hour drive in janesville. in madison we have bumper to bumpers, checker, and a weaver out in mcfarland (if its still there). my friend has an obd scanner, but hes kinda undependable with schedules and owes me money on top of that.

do you think the dealership would find themselves responsible for emissions here in wisconsin? it was a used car after all. last time i took it in they only replaced the fuel pressure regulator, and didnt even look at my leaky trunk because "they werent doing bodywork that day" and the salesman knew that im only home maybe once a month. the warranty on it expired on the 12th, and it was "drivetrain" only. ive put maybe 500 miles on it since i took in it, and the lights been on ever since. im going to call them up tomorrow and probably go down there and be loud and adamant that they should fix it.

im averaging 19 mpg, even on the highway. my dads 2000 elantra is too, and you can feel the "bucking" when you push on the gas, but hes got no cel on...hes averaging around 19mpg too. he just bought it and wants to trade it for an older rav4 :blech: or a rodeo "sport" aka amigo haha.

and i havent got a chance to even go underneath the car yet, what with it being like 25 degrees and raining/snowing outside, or me being at school. when springtime comes, im going to look the car over, every inch of it. i cant wait. i hate winter. ill tell you all what happens tomorrow.

there a 67, i saw it in that post that BN posted with the pictures of the codes from a book. its what i have, and its never flashed for me either, just solid on for a little bit.

also, theres an O2 sensor in the header too, so it could be either the one in that or the cat. since you said its the upstream one, then im gonna guess youre talking about the header one. just go buy a new one and see if that fixes it. idk the price of an o2 sensor, but i dont think a stock one form an auto parts store is gonna be too much?

WELL, heres the interesting details from the dealerships visits today. i bought the car from the hyundai budget lot, and they said whenever i needed service to take it there. so i brought it in, told them the light was off, and they said there was probably nothing they could do until the light comes back on. they said the factory warranty for emissions only went to 80k, my car has 125k. 2 hours later they tell me that they cant do anything with it because they dont have the "honda" computers, and that i should make an appointment with the honda dealer down the road (these dealerships are owned by the same person, i think theres 13 stores with vw, buick, etc in madison, 6 on this one block). i just walked in and said "this is what ive got. it wasnt fixed under warranty for some reason, tell me what o2 sensor i need so i can go buy one. theres no way im paying a grand for a new cat after the dealership simply cleared the code under warranty." so the one service advisor said that i was treated unfairly, and that i should make myself at home while he calls the hyundai service department. i go check out the brand new laguna blue s2k in the showroom (marked down to $30 grand--im taking donations, btw), and then he comes back with this somber tone in his voice and goes "im sorry for whats happended to you, being bounced around like that. im here to help you, and were going to get this taken care of. i made some calls, found out a lot, made some progress. im not going to let you leave until this is all taken care of. what time would you like to come in tomorrow, well fix everything for you at no cost." i almost wet myself! 2 weeks after my warranty expires and i can get the cat replaced FREE?? oh yeah! i said i could leave it there overnight (no point in driving across town and back) and that my dad could pick me up. he said to not bother him, and they gave me a voucher for a free taxi home. he said there might be a little wait for the ESD (i think thats what it was called. it measures the things that are out of their voltage range, hence it recognizes a bad o2 sensor etc), but the car should be done by 4 pm tomorrow! im so happy, and its going to rock when i get it back and it runs right and i will get better mileage FREEFREEFREE awesome. im out of words for the feeling. oh yeah, they said the one error code for the fuel pressure regulator was wrong, and it might be this (the ESD thingy) causing the light to go on, but hes never heard of it. ill definitely keeps the updates coming and try to parlay (what a word) what i learn to help the community here. goodbye cel :knock on wood:

got the car back today with a new cat, and new fpr. the service consultant said they had to re-do a lot of the work done by the hyndai dealer with the fpr and some vacuum (how do you spell the dam word?) hoses. he said the esd was still ok, and it wasnt doing anything out of the ordinary. so it seems that the fpr in combination with the vacuum hoses that was giving me the 19mpg and making the car run rough, and the cat giving me the cel. if an o2 sensor gives me problems in the future, i dont care, cause i got the cat replaced for free and an o2 sensor is a "drop in the bucket" compared. man, thecord (my name for it) runs so nice now with its clean bill of health. they even gave it a wash! see guys, there are some redeeming factors of dealerships! too bad they didnt write the prices for everything on the invoice...that would have been interesting. if an o2 sensor issue springs up after the cats break in, ill let you all know first.

damnit, i wish i lived in wisconsin, the dealerships here actually make you pay money to get things fixed!!!! hahaha. hope everything works out and no CEL's come back on.

i just got my cat replaced for free too... warranty on cat was 8yr/80K.... i had 78K... so, lucky break for me i guess

Congrats you got the stuff fixed.
Did they give you a warranty on the parts/work they did?