HondaFan76

ok so i have a 132,000 miles on my car....so i figured it was a good time to change the o2 sensors since they have never been changed. i had no check engine lights but sometimes it did take a few more cranks than normal to start it. After i replaced the sensors i drove it for about 20 miles and the check engine light came on and i was like wtf? I ran the code and it was p0420. Which is the downstream sensor. I figured since it was new it shouldn't do this. i took it back thinking it was defective and today i put the exchanged on in and it did the same thing as the other one. I am confused cuz they are both new and my cat has no signs of clogging because the car runs great. Idk what to do....any help would be greatly appreciated. Thank you

xRiCeBoYx

iTrader: (22)

Maybe now that you have a new sensor in there (not fouled up with 132k worth of exhaust gas/carbon deposits, it's reading what's actually passing through your cat, and it's seeing that your cat isn't as efficient as it should be

Your cat doesn't need to be clogged to throw a code.. just not be as efficient as the ECU wants it to be. I need to replace my cat because there's a huge dent on the bottom of mine (and it's throwing the 420 code), my car runs fine and and I'm showing no signs of clogging.

Sounds to me like you need a new kitty.

HondaFan76

dang. well that really sucks if it is the problem because i just replaced it like 30,000 miles ago. So idk why the cat would be less effecient.....it is a aftermarket oem replacement though....would that have anything to do with it?

Nosepicker

What is the aftermarket replacement? The 02 sensor or the cat?

xRiCeBoYx

iTrader: (22)

I wasn't saying new o2 sensors would make your cat less efficient..

It was more of a "I wonder if the old o2 sensors were fouled to the point where it was reading less emissions, making your car think the cat was working efficiently. But now that you've put new sensors in, it's reading what is actually flowing through your exhaust, showing your cat isn't as efficient as it should be"

but yes, aftermarket cat or o2 sensors?

lowlife9

try resetting your ecu and see if the light stays off you could also try the spark plug defouler trick.

remmick69

iTrader: (5)

ya do the defouler thingy

HondaFan76

my bad i ment aftermarket cat. lol. I have reset my ecu about 5 times in the last three days and it keeps comin back. last night i did the defouler thing and so far so good.....today i drove about 60 miles and no check engine light.

xRiCeBoYx

iTrader: (22)

Is it an actual cat? or is it a test pipe disguised as a cat?

HLC

um, O2 sensors don't affect start up, i believe. so yea.

gearbox

iTrader: (95)

does the code mean bad 02 sensor or low catalyst efficiency? there is a difference. if you got oem plug in type sensors, they should work. these cars hate universal sensors and will not run right. and be sure you do not confuse upstream and downstream. they are not the same sensor.

lowlife9

and just to put your mind at ease i just recently passed emissions with no cat and a defouler.

xRiCeBoYx

iTrader: (22)

P0420 is "Catalyst system below efficiency threshold"

only reason I know that off hand is because that's the only locked in code I have right now.. lol

lazlong

You need a diagnostic tool you can take with you in the car.
1. Reset ECM with the diagnostic tool
2. Turn ignition OFF and pull the #6 underhood fuse for 10 seconds
3. Use this to set readiness codes and find out if you need a new kitty:

(Sorry about pic quality, using phone and have shaky hands. lol.)
TWC=cat convertor
Good luck.

HondaFan76

no it is a oem replacement cat.....just not a honda cat. but it looks exactly the same. and yea they are the bosch oem type sensors......i know that i dont have the upstream and downstream ones mixed up. lol. I ended up just doing the o2 simulator with the new sensor and ive had no problems since. Thanks for the help guys!

gearbox

iTrader: (95)

i bet aftermarket cats skimp on the expensive metals inside so they dont work as well. thats one reason why aftermarket cats are illegal in california. they dont work as well as oem to clean the exhaust.