Thread: p0420 02 gtp

ive had this code for some time now with no problems...recently the car feels bogged down when i go WOT and running really rich. 02 sensors have never been replaced (120,000 miles on car)and i have a 3 in zzp DP with magnaflow high flow cat. i basically just need some input from other ppl who have had this prob before so i know if i need to take off my downpipe and get some of the carbon out, or just replace 02 sensors. thanks in advance.

that code is basically telling you that your cat is bad, or in your case, not good enough

edit: if you go back to the webpage where you bought your downpipe, at the bottom, ZZP tells you theres a chance this might happen. in fact it says specifically
"The downpipes we sell come with a Magnaflow high performance converter. It is OBD2 compliant but it is on the very edge of being so. The PO 420 code is set when the rear bank O2 sensor PCM thinks that the converter is not doing a good enough job. Because the stock cat is so thick and dense, it can still function when the tune is off or it gets old. The Magnaflow isn't like that."

Originally Posted by Jakegday

that code is basically telling you that your cat is bad, or in your case, not good enough

edit: if you go back to the webpage where you bought your downpipe, at the bottom, ZZP tells you theres a chance this might happen. in fact it says specifically
"The downpipes we sell come with a Magnaflow high performance converter. It is OBD2 compliant but it is on the very edge of being so. The PO 420 code is set when the rear bank O2 sensor PCM thinks that the converter is not doing a good enough job. Because the stock cat is so thick and dense, it can still function when the tune is off or it gets old. The Magnaflow isn't like that."

well i bought it used off here awhile back... is there a fix for it other than deleting the code?

yes, you could buy an o2 emulator.

well i started thinking about it, i have a 160 thermo that was tuned into my pcm...however i had the pcm flashed and lost my tune. zzp's site says that a colder thermo and an old stock 02 sensor can cause the code. could a new stock thermo and a new downstream 02 sensor possibly fix the problem?

i suppose it could..... it seems kinda coincidental that out of all the codes a bad o2 sensor could throw, its throwing the exact one that we already have an explanation for ....
if your downstream sensor is indeed bad and you put a new one on, you'll most likely get that code again since your new good sensor will be picking up that your magnaflow cat isnt doing a great job, like zzp said

well i swapped out thermo and downstream sensor, car runs alotttt better now with no unburned fuel smell. cleared code, so now ill wait and see what happens.

p0420 is useless. You can get the code deleted out of the pcm all together, if you have a tuner, or you can have a vendor do it.

cant u just plug the rear O2 port since all it is really reading is that gasses or whatever from the cat?

yea, well it ran most of the day without any probs yesterday... last night it popped a code, not sure what yet as i havent scanned it. pretty sure its the p0420 cuz it was running a little rich again. Can any tuner delete this code? or like bptech said can i jus unplug sensor for a temporary fix?

Originally Posted by gtpinsc

yea, well it ran most of the day without any probs yesterday... last night it popped a code, not sure what yet as i havent scanned it. pretty sure its the p0420 cuz it was running a little rich again. Can any tuner delete this code? or like bptech said can i jus unplug sensor for a temporary fix?

I would think the light would constantly be on if it was completely un-pluged. You can just delete it with a tuner.

Yes any tuner can. the advantage of a tuner is u can uncheck the box to make it to where it wont set a DTC

cool there is a shop in town that specializes in mustangs, maybe they can delete it for me.

Originally Posted by 99prixgt

I would think the light would constantly be on if it was completely un-pluged. You can just delete it with a tuner.

the light i wouldnt to much worry about, my car is running really rich right now... anything that would stop the unburnt smell of gas would be helpful until i get code deleted.

sooo.... arent you concerned about fixing whatevers making it run rich??

after i deleted my cat, i left my o2 sensor plugged in. i got the 420 code roughly every 4-5 days, i tried unplugging the sensor and i got the code for the missing sensor every day. it was just more conveinient leaving it plugged in

Its only running rich when the code is present. the way i was thinking and correct me if im wrong if there is no code to be thrown then i wont be running rich anymore.

if you're running too rich the front o2 sensor will pick that up and throw a p0172 code. are you getting that code?? if your rear sensor senses that your cat is inefficient it will throw a p0420 code.

which code are you getting?

edit: also, an inefficient cat, or no cat, will cause the smell of unburnt gas, i know this from experience.... i dont have a cat... that smell doesnt necessarily mean your running rich. is that the only thing making you think you're running rich?

the only code present is p0420, the car is sluggish at times...other times it runs fine. when it is acting sluggish it seems like my shift points are acting funny... what are some ther things if any that can cause this code? vac leaks etc etc. its a really stronggggggg smell of unburnt gas.

let me kinda give u some insight on this car... i bought it about 2 years ago, i went through 3 trannys in that time. dealership came to find that i had a 99 pcm in a 2002. so they flashed it back to stock. im pretty sure this car was demodded from 3.25. also pretty sure it has autolite 104's in it. my only mods are CAI and a downpipe. if those 104's are still in would they perform ok with a stock tune?

Originally Posted by gtpinsc

let me kinda give u some insight on this car... i bought it about 2 years ago, i went through 3 trannys in that time. dealership came to find that i had a 99 pcm in a 2002. so they flashed it back to stock. im pretty sure this car was demodded from 3.25. also pretty sure it has autolite 104's in it. my only mods are CAI and a downpipe. if those 104's are still in would they perform ok with a stock tune?

typically, 104's are used on 3.4 setups, and below that 103's etc etc...

You could try running stock equivalent plugs. You say it feels sluggish at times, How old is the catalytic converter on the downpipe? It might be prairie dogging.