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Old 06-11-2009, 11:53 AM   #1
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Default 99 Civic EX - P0420 Code

Hi All,

I just replaced my upstream and downstream O2 sensors and now I'm getting a P0420 code. I had my catalytic converter replaced a little more than a year ago, so I'm hesitant to have it replaced again. The car does have some issues when cold. When accelerating from a dead stop, the car stumbles (has new plugs and wires) and doesn't accelerate like it should. I think the P0420 code may be somehow related to the stumbling issue. Anybody experienced something along these lines before? Any suggestions on what to look at first?

Thanks
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Old 06-11-2009, 12:08 PM   #2
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Default Re: 99 Civic EX - P0420 Code

A rich running engine can kill a cat pretty quickly. It's best to fix O2 sensor issues first and then replace the cat.

If the cat turns out to be fine, then look for an exhaust leak.
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Old 06-11-2009, 12:18 PM   #3
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Default Re: 99 Civic EX - P0420 Code

I just replaced both O2 sensors about a week ago. Now I have the CEL and P0420 code. I'll check for exhaust leaks.
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Old 06-11-2009, 12:40 PM   #4
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Default Re: 99 Civic EX - P0420 Code

do u have an exhaust on your honda? because ever since i got my exhaust i can't get that damn code to go away and i know my cat and o2 sensors are good
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Old 06-11-2009, 12:44 PM   #5
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Default Re: 99 Civic EX - P0420 Code

Nope, the exhaust is an OEM replacement. My car is overdue on the inspection, so I'm trying to get things in order so it'll pass.
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Old 06-12-2009, 05:12 AM   #6
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Default Re: 99 Civic EX - P0420 Code

So I pulled the backup (7.5A) fuse last night to reset the computer. After just about 10 miles the CEL is back on. Now it seems to hesitate and stumble a bit more. Could the new O2 sensors be bad? I'm going to grab the CEL code tonight when I get home.
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Old 06-12-2009, 06:09 AM   #7
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Default Re: 99 Civic EX - P0420 Code

Anybody? Could a bad coil result in enough unburned fuel to trigger a P0420?
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Old 06-14-2009, 07:41 PM   #8
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Default Re: 99 Civic EX - P0420 Code

Hello everybody. I have the same problem (1998 LX) and I think it's most likely the cat as the ECU indicates. Through some research on the net I found this TSB which might aid you as you mention the coil. It involves crimping the connectors at the igniter terminals.

http://www.tegger.com/hondafaq/p0420/A05-027.pdf

If it was the sensors it the ECU would throw more codes. I have an aftermarket 2 piece (flanged) exhaust manifold / catalytic converter and I myself am wondering if this two piece is better than a one piece unit. I've also heard of a bad ECT (engine coolant temperature) sensor that could throw this code but again wouldn't this throw a different code?
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0420, 1998, 2003, 99, accelerating, cat, cel, civic, code, converter, honda, hondatech, mdx, p0420, stumbling, tsb

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