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P0420 - How to proceed?

 
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Old 01-21-2010, 07:30 PM
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P0420 - How to proceed?

Preliminary info: 99 Honda Prelude, VTEC, Automatic (4-speed), 77k miles.

While driving back to work today from lunch, I noticed my CEL came on. Cursing my luck, I pulled into the parking lot, found a paper clip in my car, and used the MIL to pull the error code. After looking it up on troublecodes.net, I found that the error code indicated was: P0420 - Catalyst System Efficiency Below Threshold.

I pray to the gods that my rear O2 sensor bit the dust as I'd much rather replace it than the CAT. Lucky for me, I have a codelink device and laptop handy to hook up to the OBD port. I look at the live data coming from the sensors, and it looks like H02S S2 (rear O2 sensor, I'd say?) has a very pronounced sinusoidal shape, only slightly smaller than the graph for S1. My co-workers say this means two things: 1) the O2 sensor is flaky and reporting bad data, or 2) the O2 sensor is working fine and points to a dead/dieing CAT.

Any suggestions as to how I should proceed? Replace rear O2 sensor, reset ECU, and then wait and see if the same trouble code comes back? Skip straight to buying a new CAT? Both?

Thanks in advance for any help! Also would appreciate good brand names to look for when buying either an O2 sensor or a CAT for my 'lude... all I've heard so far is stay away from Bosch.
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Old 01-21-2010, 10:05 PM
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Re: P0420 - How to proceed?

I work at a dealership and in my experience P0420 Almost Always means a bad cat on all cars, sorry to tell you. And if the cat was bad for long enough it can wipe out the rear HO2S also, and you will prob have to end up replacing both. My 98 lude has had this code for a while off and on. I keep clearing the code and the light comes back on after a month or two. Im thinking about getting one of those open cat test pipes that are all over ebay for like $15. For that money its worth a shot.
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Old 01-22-2010, 12:46 AM
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Re: P0420 - How to proceed?

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Originally Posted by CTLude View Post
I work at a dealership and in my experience P0420 Almost Always means a bad cat on all cars, sorry to tell you.
That's what I've been getting. :(

I'm really tempted to try a DIY replacement of my cat at this point; I've got a can of PB blaster, plenty of socket wrenches, and a lift rack at my disposal at work.

Any recommendations of good brands to look for? Eastern? Walker? Magnaflow seems to be quite pricey, unless I'm looking in the wrong place...
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Old 01-22-2010, 01:16 AM
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Re: P0420 - How to proceed?

if you are looking for O2 sensors, denso is the way to go.
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Old 01-22-2010, 03:21 AM
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Re: P0420 - How to proceed?

Ive had this exact same code come up on my 97 type sh...unfortunately it came out to be the cat...as CTLude said its 99% of the time a bad cat...i had to replace the entire cat back system and rear o2 sensor...take it to your local dealership and have them analyze the problem...hope this helps
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Old 01-22-2010, 10:24 AM
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Re: P0420 - How to proceed?

Yep. Definitely the cat.
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Old 01-22-2010, 10:30 AM
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Re: P0420 - How to proceed?

its the cat....
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Old 01-22-2010, 12:59 PM
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Re: P0420 - How to proceed?

I would test your downstream O2 first, these do go bad, pull out the O2, put a torch (propane) to it, measure voltage, if below like .9, then the 02 is bad. Or just throw an O2 in there, see if that does it, if not, then you need a cat. My advise is to go ahead and spend the extra 20-30 bucks for th OE O2 instead of the universal O2, those are a nightmare. 420 means either your downstream O2 is bad or that the cat that it monitors is no good. Before you go and buy a cat I would try that downstream O2 might save ya a few bucks.

Last edited by Ezdelaft; 01-22-2010 at 03:33 PM.
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Old 01-25-2010, 04:33 PM
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Re: P0420 - How to proceed?

we sometimes sublet our cars to meinike and they just cut out the old cat and weld in a new aftermarket one and they do the whole job for about $100. That way we dont have to pay $400-500 for an oem one from the dealer.
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Old 01-27-2010, 03:31 PM
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Re: P0420 - How to proceed?

$400-500? Ouch.. that *has* to be a little high. The magnaflow OEM direct fit ones I found were in the range of $200-300 at most ($100-150 on ebay).

Anyway, I decided to reset the code and see if it comes back; I figured if the cat really is bad, then it should just pop back on after a few driving cycles. I've driven it about 5 times now (only counting instances where I drive it long enough to go into a closed loop, of course) and no codes have shown up yet... I'm hoping it was just a bad batch of gas that wasn't burning right or something along those lines.
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Old 01-27-2010, 05:06 PM
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Re: P0420 - How to proceed?

IMO, when it comes back on I think you should try replacing just the downstream O2 sensor first, in my experience it usually solves the problem. I have even watched obd2 civics run test pipe and no code before! So if you dont feel like doing tests on the O2 to verify if it is bad then just replace it , clear codes, see if it comes back, then you know for sure you need a cat.
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Old 01-27-2010, 07:00 PM
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Re: P0420 - How to proceed?

I did connect a OBDII diag tool to my laptop and viewed graphs of the up- and down-stream O2 sensors. The downstream O2 was showing a very pronounced sinusoidal shape (only slightly less in magnitude than what the upstream was showing). Guess that doesn't rule out a flaky/malfunctioning sensor, though.
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Old 01-28-2010, 06:48 PM
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Re: P0420 - How to proceed?

I hate this code so bad!!!! There should never have been the 2nd 02 sensor, it is only and emission alert to annoy you. I have fixed them in the past by just replacing the 02 sensor with one from a junkyard, dont forget most civics have the same sensor. Most I have ever paid is 8 dollars for one. Right now I am trying a different attack with a custom exhaust and a magnaflow cat. I WISH I NEVER tried it, I am about to cut the damn magna flow out, throw away the sensor and hook up an eliminator.
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Old 01-29-2010, 10:14 PM
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Re: P0420 - How to proceed?

Good luck! I swapped my rear O2 after repeatedly clearing a 420 code. I also tried the plug defouler on it as well. Neither worked for long if at all based on how infrequent my CEL happened. I eventually found a cat from another Lude and no trouble since. The donor owner wouldn't remove it as agreed so I had to saw it off. Hence I spent a loooong time boring the old bolts out of the donor cat. Torching and breaker bars didnt help.
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Old 01-29-2010, 10:16 PM
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Re: P0420 - How to proceed?

BTW b/c Im required to have emissions I went with an OEM cat b/c every brand of aftermarket cat had at least one person say it still threw a CEL after install. Again wish you the best!
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