P0420

[chewie_jrc]

Mods: DC cai and header.

I was throwing at P0134 and P0420 code. I replaced the primary 02 sensor with a denso. CELs went away. After around 200 miles, im seeing a P0420 again. What gives? Just had a smog (failed because i dont have CARB sticker for CAI but I did pass emissions) and the emissions were:

HC - 4 avg, 21/7 meas
CO - .01 ave, .10/.03 meas
NO - 16 ave, 51/46 meas

Looks like the catalytic converter is functioning...WTF?

[03Ep3617]

X2

did same exact thing to mine

what is this about ?

[RHCP0801]

p0420 means you need a new cat, has nothing to do with the sensor you just replaced...that fixed the p0134. If you have a test pipe in there than you need an 02 simulator, if the stock cat is in there than its bad causing that code.

[chewie_jrc]

p0420 means you need a new cat, has nothing to do with the sensor you just replaced...that fixed the p0134. If you have a test pipe in there than you need an 02 simulator, if the stock cat is in there than its bad causing that code.

yes, I realize p0420 is a "faulty cat" code. IT is my understanding that a bad primary 02 sensor can erroneously cause the computer to think the cat is bad (poor readings between front and rear sensor, etc.).

Is the cat really bad if I'm passing my emissions by such a large margin? It seems like its doing its job...

Update: I reset the CEL again; so far it has not returned but we shall see.

[Heitzke]

P0420 is triggered by the secondary 02, the sensor that basically detects if the cat is working. Since your car seems to be fine, it's likely that sensor. They're annoying though, sometimes it might not return after resetting the ecu.

[03Ep3617]

Yeah my next guess was going to be that our 2nd o2 sensor is faulty, or as the P0420 states reading below threshold.

I have heard that when replacing o2 sensors they should be replaced in pairs, as to the new sensor may read more efficiently and the used 2nd o2 in our case is more contaminated from past use.

We probably skimped out not trying to dish out so much money on two sensors at once and now are dealing with this dreaded P0420...

[chewie_jrc]

Makes sense. I'll order a new secondary 02 sensor here in a few. Anybody have the item # for our desno secondary 02? (2004 Si)

[talonXracer]

There is no such thing as a matched pair of o2 sensors, nor a requirement for them to be matched.

A secondary o2 sensor could be skewed enough that it is indicating a faulty cat, when in fact, it is it's own internal resistance that is faulty.

Do make sure to reset the ECU after changing any part that can impact the operation of the fuel/ignition systems.

[DA9_GSR]

As a automotive technician, I will tell you that you can change all the parts you want, the P0420 will continue to come back if you don't replace the catalytic converter. It's neither of the sensors. If either of the H02S' had a short (ie: connector pins touching one another), the ECU would see a constant 12V, which would trigger a completely different code. If the H02S' had an open circuit (ie: broken wire/s), the ECU would see a constant 0V which would trigger a different code. Because the ECU is seeing something a little off normal, it's tripping the P0420 code which could only mean one thing, the catalytic converter.

This is a common thing with Honda since the beginning of time due to Honda's nature of using a three way catalytic converter and running their engines leaner (which also means hotter) than most other manufacturers. Running their engines leaner lower the HC and CO emissions. However, it will raise your NO emissions. That is where the three way catalytic converters come in. They have a "bed" in the catalytic that converts the NO emission into nitrogen and oxygen molecules (same things that make up our atmosphere).

Change the catalytic converter, and get the Honda part. Aftermarket ones use different materials that don't work as well. Honda uses Platinum and Rhodium for their catalytics, which are exteremly rare metals and now cost a shitload of money. Well worth it in my opinion.

[RHCP0801]

i have been a honda tech for 7 years and told him to change the cat, he isnt listening to me or you so we are just wasting our time...let him spend the money on another sensor

[Onasty]

:hay:i have a high flow cat for sale

[AzNCrAzYcOoLeR]

I changed the cat with a magnaflow cat, and i still get the light. Both sensors have been changed too

Any other ideas for me? Should I just do a 02 trick?

[DA9_GSR]

I changed the cat with a magnaflow cat, and i still get the light. Both sensors have been changed too

Any other ideas for me? Should I just do a 02 trick?

Pray to god it passes the Drive Clean emissions testing for Ontario. If not, you'll need a Honda catalytic converter. The aftermarket doesn't get access to platinum and rhodium metals. So they use other crap to make up a "catalytic converter". Usually they might pass and they might not, it's a roll of the dice.

P0420 is a catalytic converter fault and nothing else
In Honda's diagnostic procedure, they make you erase the permanent code and check for it again in the temporary codes. If it's there, they want you to replace the catalytic converter. If not, they make you drive the car in a certain manner to "trip" the code. If it doesn't trip, they tell you to erase all codes and chalk it up to a fluke. I've had my P0420 code come on twice in the last year. I have no mods and 174'000 km (108'000 miles). I've always erased it and it doesn't usually come back for long while. I've even had emissions testing in between and it passes pretty well.

[AzNCrAzYcOoLeR]

Thanks. I did emissions here ...and it passed ... almost fail though. 02 trick worked as well.

Honda Dealers around here want 2k installed. I might have to get one from the states .. $700

[AKEP]

go ask for a courtesy test to see how it does. smog techs are usually pretty cool about that.

[AKEP]

how is my post edited by cow @ 7:28 (nov 26 2008) when i just posted it @ 7:53???

[cvicep3]

I recently had the CEL came on. Checked by my friend, mechanic and it was P0420. Reset it and it came back on again after a week. What should I do?

[usedep3]

replace the cat. if you have less than 80k the dealership might replace it for free under warranty. at least mine did

[cvicep3]

Too bad can't get it replaced under warranty. It has 118k.


replace the cat. if you have less than 80k the dealership might replace it for free under warranty. at least mine did

[SkareKrow]

p0420 means you need a new cat, has nothing to do with the sensor you just replaced...that fixed the p0134. If you have a test pipe in there than you need an 02 simulator, if the stock cat is in there than its bad causing that code.

Where to get an 02 Simulator? Thanks in advance.
I am getting the same code. x2 Its my cat I know because I got it checked at Honda. Im just going to "Test Pipe It" But where would I be able to get an 02 Simulator?

[cvicep3]

My CEL went away this morning by itself when I drove to work. Not sure why.