P0420

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Yup

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It's a 2002 Honda Civic Si K20A3 engine. It's thrown a P0420 twice in the last couple of months. I did a valve adjustment a few months ago and I'm not sure if that's a coincidence. The car has 175K with original O2 sensors as well. I was thinking I'd try replacing the O2 sensors first because a cat is ridiculously expensive. Any other items that might throw this code on this car (in case there's something that commonly causes it on this engine)?
 
Originally Posted By: Roadkingnc
Put a spark plug nonfouler on your catalyst monitor
And this will repair the car how? I for one do not like to cover up a problem.
 
At about 100,000 miles I replaced my Honda cat, due to the P0420 code, with a Magnaflow twice, and only had them fail in a short time. I tried both the OEM grade Maganaflow and the regular Magnaflow in my Honda and both clicked on the P0420 code within 10,000 miles. I spoke to a couple of dealer techs and they told me that I need to use an OEM Honda cat and to use OEM spark plugs that will keep the combustion chamber cleaner than aftermarket plugs thus allowing a longer cat life. I don't know if this theory is all true but since I switched the cat to a new OEM Honda, which cost me $1,200 for the part only, I have had no problems and now have 180,000 miles.
 
Originally Posted By: Kool1
I switched the cat to a new OEM Honda, which cost me $1,200 for the part only


That is a world of pain...
 
I takes the sensor out of the exhaust stream keeping the sensor from switching from rich to lean due to the defective catalyst. The proper and expensive repair would be to replace the defective catalyst as it is now chemically inactive.
 
Originally Posted By: Bluestream
Originally Posted By: Kool1
I switched the cat to a new OEM Honda, which cost me $1,200 for the part only


That is a world of pain...


Someone stole the cat converter off my aunt's 02 Accord recently while she was on vacation. The dealer replaced it. About a month prior she canceled her comprehensive rider on her car insurance policy. Which would have covered it. She was upset about that one.
 
If you are going to replace the cat, i'd replace the upstream sensors, they get slow with age. Also replace the plugs and if applicable the wires too. I don't think its necessary to replace the post cat sensor as it only monitors, it does not affect the stoichiometric ratio.
 
My son's girlfriend had a Honda SUV of some sort. Someone stole the cat and they totaled the car.
smile.gif


My son put an Eastern cat on, it still gets the cat efficacy code.
 
I've read some other forums and OEM cat seems like the only way to go. And the $1000+ sounds about right. Some say say a bad O2 can cause the code...oddly, many say the downstream, not the upstream. Both times it came on were when the engine was under heavy load going up hills (takes effort to shift). I thought that might be something to do with it to. If it comes back I'll do O2 and then, hopefully not, cat. Or just reset the code and trade the [censored] thing in. As bulletproof as Hondas supposedly are, mine has had minor annoyances. I might go with a cheap Chevy or Ford compact instead of a Fit or Civic for $6-8k more. 6-8k could easily pay for the extra maintenance on a Chevy or ford. Ha!
 
If you get some sort of data link OBDII reader you can monitor the O2s voltage. It will tell the tale of the 2nd O2 as it doesn't change as much as the primary. Also the 2nd O2 will usually run, when in closed loop and steady load, at near the primary O2 high level ~.750v.
 
Originally Posted By: Yup
I've read some other forums and OEM cat seems like the only way to go. And the $1000+ sounds about right. Some say say a bad O2 can cause the code...oddly, many say the downstream, not the upstream. Both times it came on were when the engine was under heavy load going up hills (takes effort to shift). I thought that might be something to do with it to. If it comes back I'll do O2 and then, hopefully not, cat. Or just reset the code and trade the [censored] thing in. As bulletproof as Hondas supposedly are, mine has had minor annoyances. I might go with a cheap Chevy or Ford compact instead of a Fit or Civic for $6-8k more. 6-8k could easily pay for the extra maintenance on a Chevy or ford. Ha!


That's a myth about the maintenance on a Chevy anyways. I just had my first issue ever (CEL) on my 2002 Chevy Cavalier and I fixed it with a 20 dollar part. Maintenance is much cheaper on a domestic vs. import. Import parts are lots of times over 5X more depending on the brand vs. GM parts.
 
Originally Posted By: Yup
As bulletproof as Hondas supposedly are, mine has had minor annoyances. I might go with a cheap Chevy or Ford compact instead of a Fit or Civic for $6-8k more. 6-8k could easily pay for the extra maintenance on a Chevy or ford. Ha!


Having owned Hondas/Toyotas since 1994 - present, I agree with you. They all seem to have their pros and cons.
 
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