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strayheart 02-09-2009 08:51 PM

Damn P0420 code!
Alright I'll keep it short!
Honda Prelude 2000 Base Auto

Got this CEL code during the really bad winter snow time in december.
At first I thought it was just temporary due to the fact that my car is lowered and it bottoms out everywhere in the side streets!

So now the weather is much better, I dopped by the car shop to get it checked up. First they said it's the O2 sensor... once they got the code...
so I ordered the front O2 sensor and replaced it... the light came back on that very night. Few days later I returned and they said that it is probably the rear O2 sensor. Now I got that replaced and they are stumped as to what is triggering the code!

I had a fractured/clogged cat last autumn, and so I got a new magnalfow direct fit cat as a replacement. The cat worked wonderfully and so nothing happened until late december during the snow death period.

Checked for Leaks
Inspected the Headers/Exhaust for any cracks or damages
Changed both the O2 sensor
Inspected the cat for any mal-installation

so what should I do now? I am getting soo annoyed of this damn P0420 code!
Someone Help!!

Nvasion 02-09-2009 08:54 PM

clean your maf sensor?

strayheart 02-09-2009 08:59 PM

ummm.. i'm not a pro with cars...
maf sensor? being what?

bootymac 02-09-2009 09:12 PM


Originally Posted by strayheart (Post 6272637)
I had a fractured/clogged cat last autumn, and so I got a new magnalfow direct fit cat as a replacement. The cat worked wonderfully and so nothing happened until late december during the snow death period.

If you've replaced both O2 sensors, then this is probably your problem. The aftermarket catalytic converter is not of the same standards as the OEM catalytic converter, so the rear O2 sensor is reading higher numbers of emissions. The ECU interprets this as a faulty catalytic converter.

One trick is to space out the rear O2 sensor to lower its readings, which will hopefully keep the CEL off. The rear O2 sensor is purely for emission purposes, so this won't have any effects on drivability

strayheart 02-09-2009 09:16 PM

how exactly would I space out the rear sensor?

BEEB 02-09-2009 09:31 PM

you can DIY with spark plug spacer or buy aftermarket O2 sensor spacers.

strayheart 02-09-2009 09:35 PM

will this cause a problem for aircare later down the road if I use this trick?

Nvasion 02-09-2009 10:54 PM

mass air flow

Puck Luck 02-09-2009 11:43 PM

Uses a manifold air pressure (map)
doesnt have a mass

Puck Luck 02-09-2009 11:52 PM

I talked to honda techline a while back about a car that had 0420 problems. I replaced the converter but it came back about 10,000 kms later with the converter guts melted. Something was obviously killing the converter. Cant remember what i found though. Sorry.
Like bootymac says, aftermarket converters arent the same quality as oem converters.
Hows the car running? Not misfiring is it?

godwin 02-10-2009 12:06 AM

It could be the cat is dead again.. your car had been bottoming out in the snow couldn't do the cat any favours.

Why don't your shop really test the gas composition and figure out what's wrong in the first place before they randomly replacing stuff? (besides the obvious that they are charging you labour and profit for parts eg for the O2 sensor changes).

strayheart 02-10-2009 03:49 AM

the car runs fine and no loss of power... but it's really annoying to have this BRIGHT amber "Check Engine" Light blinding you as you cruise in the night!

godwin - yeah thats what i thought at first but we have inspected and confirmed that the cat is fine. They are very close friends so they barely charge me labour at all, and parts I just find on my own and bring it to them. I'm not sure if it's because they are lazy or they arent as experienced with Hondas, whichever the case is.. what should I do
Anyone on RS who lives in vancouver and is a honda pro?! If you're out there... HELP!!!!

Puck Luck 02-10-2009 03:39 PM

Is a good idea to have the gas comp checked out.
I remember techline saying to have a look at the fuel trim levels. It can give you a look at the short history of how its running. (too lean or too rich) As opposed to 02 sensor showing it in real time.
Also while its running, compare the primary and secondary 02 readings. Secondary 02 typiacly should show a constant .8 volts (approx) at high idle when the converter and engine is warmed up.
If the primary and secondary readings/patterns are the same or very simular then is definatley problem in converter. Next queston would be, why is the converter failing.
Did they have a look inside the converter too see if the guts have been damaged?

off topic slightly, I'm running without a converter at the moment. Just a straght pipe. I have a 02 sensor simulator in place of the seconday 02 sensor and have been doing so for the past 2 years without engine light problems. Obviously the simulator (which happens to be easily available online) isn't the proper fix, but does keep the light out.

strayheart 02-11-2009 11:44 AM

hmmm alright maybe I should just go to a different more reliable shop >.<
can someone reccommend me a place? or anyone on here is a mechanic themself and can help this noob out?! T.T

godwin 02-11-2009 04:20 PM

Your cat doesn't have to display external damage to loose its effectiveness.. it can melt internally or you have too much unburnt fuel/gunk and it can kill the cat.

Go to any of the sponsors in this list for service:

The person/shop doesn't have to be a Honda pro honestly, it sounds like something fundamental.


Originally Posted by strayheart (Post 6275580)
hmmm alright maybe I should just go to a different more reliable shop >.<
can someone reccommend me a place? or anyone on here is a mechanic themself and can help this noob out?! T.T

!oHenry 02-11-2009 07:36 PM

Did you check your timings? Oh and i'm not sure if this is enough to emit a CEL code but you don't use regular gas right?

strayheart 02-11-2009 07:50 PM

nope i use 94 at chevron at all times

strayheart 02-17-2009 11:21 PM

ok so I took it to Team Work on kingsway fo Kevin to deal with it...
after inspection he said that there was a minor leak that MAY cause the engine code to pop up. So he took out the cat and everything to inspect the inside to make sure nothing was damaged. He replaced the gaskets to ensure that here is no gaps or anything. He fixed the leak and also changed my oil pressure sending unit due to mine being damage and cause oil leak.

Everything seemed cool and I thought maybe this time it's gone for good.. then again this code just popped up. (on the same day)

so now I have to bring it back to team work for them to take it for a drive with monitor for approx 1.5 hour to see the readings of all the units in the car and see what is causing this.
Is this problem common? cuz it sounds like its going to get very expensive and also this is very strange, Kevin reccomended me to get a hi-flow cat to fix the problem =/
All I hear is the sound of money *chit ching* going out of my wallet >.<

Klobbersaurus 02-18-2009 05:50 PM

a good shop wouldnt give you a may, they should give you a definite answer, i mean, think about it this way, if a regular aftermarket cat is not efficient, how much more efficient would a high flow cat be, you will just be throwing parts at it over and over again

strayheart 02-18-2009 06:42 PM

so as it stand right now...
should I go try a different shop? if so plz rec. me one.
I kinda of have faith in Team Work tho since I have good words from close friends and college that this is the place to go.
However I'm more interested in the best treatment for my car in general so any suggestions will be highly appreciated

Klobbersaurus 02-18-2009 07:48 PM

well the thing is, it could be that you need a new cat, have they checked to see if the cat is working properly? checking that is actually doing what its supposed to, checking the emissions coming out of the tail pipe

the place i work at charges $150 diag to start but if you think you need a new cat, would you want to spend 150 for us to tell you what you already sort of know?

strayheart 02-18-2009 09:12 PM

thats the thing, they already checked the cat and tested it.. and it seems to be fine, they are not sure why its still throwing the code.

wacko604 02-19-2009 01:51 AM

What's the mileage on the car now? Has it ever had a misfire? (A missfire will cause damage to the cat) What cat is in the car right now? A OEM Dealer Cat from honda should fix this problem. I wouldn't reconmend a high flow cat or any cheap aftermarket one. If you put a cheap on in the code will come back up.

strayheart 02-19-2009 12:04 PM

the car is at 166km right now and there is no misfire
the cat right now is a OBDII magnaflow direct fit for prelude 97+
It's not a Hi-Flow cat, just a basic oem replica. The main reason for myself and most to not go to OEM honda cat is it cost like $700 for it... which is quite insane >.<

Puck Luck 02-19-2009 03:38 PM

An oem converter from honda i think is more than 700. Closer to 1000. if the converter you have in it now is fairly new, then regardless of wheater its oem or aftermarket, is sounds like its burning out. You said they checked and tested it. I'm curuios to know how they tested it. Did they moniter the secondary o2 sensor reading? did they do a tail pipe and pre-cat sample test? (Aircare requires the cat to be 80% efficiency or better. there should be roughly .5% co and roughly 50-60 hc before cat.)

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