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Thread: P0420 Code AGAIN!!!

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    P0420 Code AGAIN!!!

    Okay, this is my least favorite number. This is my third time recieving this code within 9 months. The first time I replaced the o2 sensor before the monifold. I then got the same error three months after and replaced my other oxygen sensor along with the previous one. Therefore I have had two new o2 sensors in my car for the last 3 months. They are the bosch universal ones. I read after installing that those aren't the best to get, but they are the cheapest. Anyway, I am now wondering if my cat is bad and what I should do about these stinkin o2 sensors. Please help me fix this problem so it won't come back again.

    Thanks, Andrew

  2. #2
    lots and lots of fail 98CoupeV6's Avatar
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    Why are you replacing O2 sensors? P0420 is a catalyst efficiency code.

    Edit: In my AV6, anyway. What engine do you have?
    2003 San Marino Red Accord Coupe EX-V6 6MT
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    Its a V6

    The code states that "Catalyst System Efficiency Below Threshold", I was told this was a problem with the o2 sensors from reading other forums. Problem seemed to go away after I replaced the sensors but then came right back months later. What should I do??

    Oh yeah, its also automatic. Same year as yours too (1998). Do I need to replace the cat converter? Tranny is also starting to slip a bit, errrr

  4. #4
    lots and lots of fail 98CoupeV6's Avatar
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    Quote Originally Posted by Vladi17
    The code states that "Catalyst System Efficiency Below Threshold", I was told this was a problem with the o2 sensors from reading other forums. Problem seemed to go away after I replaced the sensors but then came right back months later. What should I do??

    Oh yeah, its also automatic. Same year as yours too (1998)
    Well Andrew, I think the only way that P0420 would come on and an O2 sensor would be the cause would be if there was another trouble code such as P137 or P141 indicating an O2 sensor problem, which would trick the computer into thinking that the catalytic converter wasn't doing its job and thus setting off P0420. My PO420 has been on for the past few months and I'm 95% sure it's the cat. I really need to replace that.

    DTC P0420: Catalyst System Efficiency Below Threshold

    1) Reset the ECM/PCM.
    2) Start the engine. Hold the engine at 3,000 rpm with no load (in Park or neutral) until the radiator fan comes on.
    3) Except F23A4 SULEV engine: Test-drive at 40 - 55 mph (64 - 88 km/h) for approx. 2 minutes . Then decelerate for at least 3 seconds with the throttle completely closed. Then reduce the vehicle speed to 35 mph (56 km/h ), and try to hold it until the scan tool indicates FAIL or PASS as the TWC test result. F23A4 SULEV engine: Test-drive at 50 - 55 mph (80 - 88 km/h) for approx. 2 minutes . Then decelerate for at least 4 seconds with the throttle completely closed. Then accelerate to 55 mph (88 km/h ), and try to hold it until the scan tool indicates FAIL or PASS as the TWC test result. Is the test result FAIL? YES - Check the three way catalytic converter (TWC). If necessary, replace the TWC. NO - Intermittent failure, system is OK at this time.

    So if P0420 comes on and it's the only code, either replace the TWC or have your Honda dealer (or someone else) test the TWC to see if it's bad or not. If there are multiple DTCs, troubleshoot those first. I only have P0420 I'll copy this in the DTC thread.
    2003 San Marino Red Accord Coupe EX-V6 6MT
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    Twc??

    Sorry, I'm a little confused by your reply. Not sure what the Three-Way Converter (TWC) is. Also not sure what F23A4 means, could you please go a little bit more into details about what all this means. Also, I don't have a scan tool, I went to autozone for the reading. Should I buy one and have it plugged in while I drive?? I'm a little confused. Thanks for your quick response and all your help.
    98 Accord Coupe V6 EX Automatic
    Detroit, MI

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    Senior Member skabone69's Avatar
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    sounds like you should try the o2 defouler trick.

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    o2 defouler trick?

    Could you be a little more specific???
    98 Accord Coupe V6 EX Automatic
    Detroit, MI

  8. #8
    lots and lots of fail 98CoupeV6's Avatar
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    Quote Originally Posted by Vladi17
    Sorry, I'm a little confused by your reply. Not sure what the Three-Way Converter (TWC) is. Also not sure what F23A4 means, could you please go a little bit more into details about what all this means. Also, I don't have a scan tool, I went to autozone for the reading. Should I buy one and have it plugged in while I drive?? I'm a little confused. Thanks for your quick response and all your help.
    The TWC is the Catalytic Converter. F23A4 is an Accord ULEV engine.

    The scan tool referred to is a Honda tool that allows them to scan your entire ECU and get all sorts of data. I believe you can purchase one but they're very expensive.
    2003 San Marino Red Accord Coupe EX-V6 6MT
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    Thanks for the help

    Okay, I think I'm gonna replace the cat converter, seems to me that has to be the problem since I replaced the both 02 sensors. Found one for around $140 (3.0 V6 Direct-Fit), if you know of cheaper please let me know.
    98 Accord Coupe V6 EX Automatic
    Detroit, MI

  10. #10
    lots and lots of fail 98CoupeV6's Avatar
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    Quote Originally Posted by Vladi17
    Okay, I think I'm gonna replace the cat converter, seems to me that has to be the problem since I replaced the both 02 sensors. Found one for around $140 (3.0 V6 Direct-Fit), if you know of cheaper please let me know.
    Yeah if I were you I'd either go OEM or Magnaflow. Magnaflows do not set off the check engine light. I have no idea who makes those $140 ebay cats, I almost bought one but I've never heard any opinions on them or even who manufactures them.
    2003 San Marino Red Accord Coupe EX-V6 6MT
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  11. #11
    Senior Member skabone69's Avatar
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    Quote Originally Posted by Vladi17
    Could you be a little more specific???
    this should explain it pretty well

    Quote Originally Posted by notoriousB
    yup, we did this for a grand total of $3.99+tax i believe!

    get this "help" part at your local parts store: 18mm defouler - comes in a pack of 2

    here's the package and the two defoulers screwed together


    i drilled out the bottom of one with a 1/2 inch drill bit - go slow, use lots of oil, it's thick

    i don't have any pictures of the one drilled out alone, but here's two pics of the final product, one from the bottom one from the top. in the second pic you can see how it was drilled. the drilliing is done to be able to fit the o2 sensor thru the 1st defouler.





    final product all installed, and skabone confirms "it works!!"



    had to persuade the heatshield slightly to clear the new longer o2 sensor but that's no problem.
    I got better milage doing this as compared to pulling the fuse for the ECU over and over again. I get about 400miles on a tank now.

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    This is the cat I'm lookin at buying, looks alright, not off ebay.
    What do you think??

    http://www.convertersexpress.com/ind...ig=&cylcount=6
    98 Accord Coupe V6 EX Automatic
    Detroit, MI

  13. #13
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    good luck with that $140 cat as you will most likely get the P0420 code again as aftermarket cat. won't burn clean enough to keep the light out as new cat. for the 2.3l motor is $2,000 for a california car as they are ULEV the V6 is cheeper but still lots of money...just use the fouler plug trick with the stock cat. as you will still need to the fouler trick for the aftermarket cat. also if you are from california it's illegal to put a aftermarket cat. on a 0BD2 car.

  14. #14
    99 Accord LX Maukio's Avatar
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    There are two O2 sensors on our car and where you got confused is you replaced the O2 sensor on our exhaust manifold. It manages the air/fuel ratio. The code you keep getting means your cat efficiency is below the acceptable threshold, which is determined by the second O2 sensor that's at the back of your cat. Don't pay for a new cat and if you do, get an OEM replacement, not some high-flow cat. You're gonna lose power if you do. I would say just do the spark plug defouler fix that Skabone posted. It's very simple and will cost probably $5.

  15. #15
    lots and lots of fail 98CoupeV6's Avatar
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    Quote Originally Posted by stereo
    good luck with that $140 cat as you will most likely get the P0420 code again as aftermarket cat. won't burn clean enough to keep the light out as new cat. for the 2.3l motor is $2,000 for a california car as they are ULEV the V6 is cheeper but still lots of money...just use the fouler plug trick with the stock cat. as you will still need to the fouler trick for the aftermarket cat. also if you are from california it's illegal to put a aftermarket cat. on a 0BD2 car.
    No, the F23A5 is the ULEV.

    Magnaflow aftermarket cats DO NOT put out check engine lights!

    Try the defouler trick though if you please, I'd like to see if it actually works for you. My opinion is that your cat is worn out, but try the $5 fix first like these guys say.
    2003 San Marino Red Accord Coupe EX-V6 6MT
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    just keep in mind that cats carry an 8 year or 80k warrenty from the factory.
    Stop asking me for radio codes!

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    Skabone69, Could you explain the "defouler" a little? Does this prevent the O2 sensor/s from fouling? Did you install on both upstream and downstream O2 sensors? How long has this solution worked for you?

    thanks
    See You Down the ROW

  18. #18
    Senior Member skabone69's Avatar
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    Quote Originally Posted by TexasHonda
    Skabone69, Could you explain the "defouler" a little? Does this prevent the O2 sensor/s from fouling? Did you install on both upstream and downstream O2 sensors? How long has this solution worked for you?

    thanks
    it pretty much tricks the secondary 02 sensor and tells the car that the emmissions are running fine. you'll only need to run the defouler for the 02 sensor that is in the cat. I've had the defouler on my car since May and I haven't had a problem. in fact I'm getting better milage now. I was getting 330-360 miles on a tank of gas, now I'm getting 390-410 miles on a tank of gas.

  19. #19
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    Quote Originally Posted by skabone69
    it pretty much tricks the secondary 02 sensor and tells the car that the emmissions are running fine. you'll only need to run the defouler for the 02 sensor that is in the cat. I've had the defouler on my car since May and I haven't had a problem. in fact I'm getting better milage now. I was getting 330-360 miles on a tank of gas, now I'm getting 390-410 miles on a tank of gas.
    Wouldn't that mess up the A/F ratio since the O2 sensor determines how much fuel is to be injected?
    2003 San Marino Red Accord Coupe EX-V6 6MT
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    The post-converter O2 sensor monitors O2 content out of the catalytic converter to assess whether converter is performing OK. It can be fooled by exhaust leaks or missing engine, and even too sensitive software assessment of sensor input can cause problems. The post-coverter sensor has no bearing on fuel-air ratio, which is controlled by pre-converter sensor input to ECU.

    regards
    See You Down the ROW

  21. #21
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    i have the same exact problem with Vladi17....it sucks..do u guys think this would couse any problem on my motor? i havnt replace my secondary 02 yet but i will this saturday..the problem is i drive 45min away from work which is 50miles....i have 3 more days on replacing it..u guys thick its safe?

  22. #22
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    Quote Originally Posted by k4sp3r
    i have the same exact problem with Vladi17....it sucks..do u guys think this would couse any problem on my motor? i havnt replace my secondary 02 yet but i will this saturday..the problem is i drive 45min away from work which is 50miles....i have 3 more days on replacing it..u guys thick its safe?
    Well I would imagine so since the post converter sensor affects nothing in your engine like tezas said. It's not like it's hooked up to the SRS and your air bags could deploy at any time, haha.

    My PO420 code has been on for 30k miles. Relax.
    2003 San Marino Red Accord Coupe EX-V6 6MT
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    Quote Originally Posted by Vladi17 View Post
    Okay, this is my least favorite number. This is my third time recieving this code within 9 months. The first time I replaced the o2 sensor before the monifold. I then got the same error three months after and replaced my other oxygen sensor along with the previous one. Therefore I have had two new o2 sensors in my car for the last 3 months. They are the bosch universal ones. I read after installing that those aren't the best to get, but they are the cheapest. Anyway, I am now wondering if my cat is bad and what I should do about these stinkin o2 sensors. Please help me fix this problem so it won't come back again.

    Thanks, Andrew
    I had the same problem with my 2000 Honda Civic EX automatic. P0420 COULD be bad 02 Sensors OR it could be the Catalytic converter! I replaced my 02 Sensor and light stayed on. So replaced the Catalytic converter and Cleaned out both 02 Sensors and problem solved!! Civic EX Cats are cheap to replace ($270). Civic LX and DX and CX have catalyctic converter connected to exhaust manifold as ONE PART and that alone costs $960 at hte DEALERSHIP!! Take it form a used car dealer, I've had many of these.

  24. #24
    MalleusMaleficarum totalimmortal's Avatar
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    Quote Originally Posted by kaoshen View Post
    I had the same problem with my 2000 Honda Civic EX automatic. P0420 COULD be bad 02 Sensors OR it could be the Catalytic converter! I replaced my 02 Sensor and light stayed on. So replaced the Catalytic converter and Cleaned out both 02 Sensors and problem solved!! Civic EX Cats are cheap to replace ($270). Civic LX and DX and CX have catalyctic converter connected to exhaust manifold as ONE PART and that alone costs $960 at hte DEALERSHIP!! Take it form a used car dealer, I've had many of these.
    Good job bringing a thread back from last year, moron.
    Stock Civic Vs. 250hp Wet Shot

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  25. #25
    lots and lots of fail 98CoupeV6's Avatar
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    Quote Originally Posted by totalimmortal View Post
    Good job bringing a thread back from last year, moron.
    At least he did a search...
    2003 San Marino Red Accord Coupe EX-V6 6MT
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    Geeeez...take a pill.

    Quote Originally Posted by totalimmortal View Post
    Good job bringing a thread back from last year, moron.
    Someone on their cycle today. Take 10 shots of anything that's 100proof and slow your roll bud.

  27. #27
    MalleusMaleficarum totalimmortal's Avatar
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    Again, this thread is from last year.

    Get out troll.
    Stock Civic Vs. 250hp Wet Shot

    "i lowered my engage point to 500rpm. I'm so hardhore, I idle in tEh tak" -e3NiNe

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    How old are you boy!

    Quote Originally Posted by totalimmortal View Post
    Again, this thread is from last year.

    Get out troll.
    Shut your hole boy and go get your passy. This fit of yours is going nowhere. Everyone is here for help and knowledge on their fav. Hondas. Tis better to bring back an old thread than start a new one. So explain to me son, what your prob? Can't wait to hear it .

    Anyway, im getting a P0420 code on my 98 Accord EX and am currently looking for a soil cat that will not throw any codes once installed. I understand Magnaflow direct fits are good. Are Bosal direct fits any good aswell? ....and please, no rage and/or idiocy with replies.
    Last edited by big2000dave; 04-18-07 at 05:28 PM.

  29. #29
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    Quote Originally Posted by big2000dave View Post
    Shut your hole boy and go get your passy. This fit of yours is going nowhere. Everyone is here for help and knowledge on their fav. Hondas. Tis better to bring back an old thread than start a new one. So explain to me son, what your prob? Can't wait to hear it .

    Anyway, im getting a P0420 code on my 98 Accord EX and am currently looking for a soil cat that will not throw any codes once installed. I understand Magnaflow direct fits are good. Are Bosal direct fits any good aswell? ....and please, no rage and/or idiocy with replies.
    wow, what a way to welcome yourself to the community what's a passy? Who the hell do you think you are being an e-thug? There is no better way to alienate yourself from a forum than to type in the tone your are typing.

    if you would have read the whole thread and my comment from 9/21, you might realize that my post could help you. might want to try out my suggestion first before you get a new cat.

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    Quote Originally Posted by skabone69 View Post
    wow, what a way to welcome yourself to the community what's a passy? Who the hell do you think you are being an e-thug? There is no better way to alienate yourself from a forum than to type in the tone your are typing.

    if you would have read the whole thread and my comment from 9/21, you might realize that my post could help you. might want to try out my suggestion first before you get a new cat.
    o i'm harmless; however, sometimes one tone deserves another. But as the saying goes, 2 wrongs don't make a right. A passy is a pacifier by the way....lol. i'll be trying your suggestion out and am thinking ahead just in case the cat does need to be replaced. Thanks for the post.

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