Error Code P0420
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Senior User
Joined: Oct 2005
Posts: 328
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From: Indianapolis, IN
My buddy has a voltmeter to test the O2 sensors so I'm going over there tomorrow to test them. We may switch them around too, we'll see. I hope it's the sensor, much cheaper as opposed to a new cat.
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Senior User
Joined: Oct 2005
Posts: 328
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From: Indianapolis, IN
Before testing the O2 sensors or cats I decided that we were going to disconnect the negative cable on the battery for about a minute and then reconnect it and see if the error code comes back. I've driven just a little over 20 miles since and the light is currently off. So I'm wondering if it was just a small mixture of a bad batch of gas thrown in there and it triggered it. Who knows. It's off now, driving like normal, so I'm happy. Thanks for all the advice guys! I knew I came to the right place for help. 
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Joined: May 2003
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From: E.Lincoln County, NC
If you've only driven 20 miles I doubt seriously that the "drive cycle" requirements for self testing has taken place. Give it to 50+ miles and then if it's still off you can take a guarded breath.
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Joined: Jan 2004
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From: allen park
Yes. The P0420 requires two errors to set off the CEL. I can confirm this, as I have a ODB2 Code reader. Whenever my CEL comes on, it's two seperate P0420 codes. I'm in the same ballpark as all of you. I'm not messing with it until it starts affecting engine performance or falling apart.
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From: Broken Arrow, OK
Originally Posted by Kijutsu
I'm not messing with it until it starts affecting engine performance or falling apart.
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From: allen park
Originally Posted by Silver Streak
That's a really poor plan these days. If the CEL is on all the time it won't be able to tell you when you have a real problem. One of my customers had his light on for years for an EVAP leak. He brought it in when it started running bad to find that he had all but destroyed his engine. If he had spent the money to fix the EVAP leak it probably would have saved him a ton of money down the road.
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Senior User
Joined: Oct 2005
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From: Indianapolis, IN
So far I've gone about 70 miles and no light on. I'll have to fill up or atleast put some gas in my truck tomorrow or I won't be going anywhere so I'll go to my normal fill up station. But so far everything looks promising.
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Senior User
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From: Indianapolis, IN
Alright, 155+ miles and no error code. I'm thinking whatever it is has passed (i hope). Thanks again for all the info guys, much appreciated. I knew I could get help here.
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Joined: Apr 2007
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i have a 2000 ford f150 with a 4.6l 4x4 i've brought it to the ford dealer 2 times know and the first was cause of my windshield leaking onto my gem and lean on bank 2 brought it there today and know i still have lean on bank 2 so they told me to change plugs on that side ,i did and it still has a rough idle but if i give it about 200 rpm it runs great ford said the o2 sensor was ok but what do you guys think is it a pluged cat.,or o2 sensor i know its not the after cat. sensor but i know the before cat sensor is the one that controls air fuel any help would be a much thanks and if i could a 24 of beer lol
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Joined: Nov 2005
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From: Central Valley, CA
Originally Posted by ranger88a
I agree with bhsuperduty, being a tech also 95% of the time it's the cat.