P0420 need advice

Is your car throwing codes or are you having a hard time passing your state or county emission test? Ask how to fix it here.
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cdewalt1985
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P0420 need advice

Post by cdewalt1985 » Fri Jun 29, 2012 11:30 pm

ok guys I know this is either cat, 02 sensors or needing a tune up. I am looking for advice on where to start. here is the backround. I have had the car for over a year now, Awhile back say 3 months ago my car threw this code, I installed a CAI and it went away for a few months, now it is back again. The car doesnt stink like the cat is clogged, it runs fine and doesnt seem to have any issues. I have put over 20k on the car and have not done a tune up. So I am pretty sure it isnt the cat, would you guys start with plugs and wires and coilpack or do you think its one of the 02 sensors? Any input would be great. thank you
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occasional demons
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Post by occasional demons » Sat Jun 30, 2012 9:41 am

It is really anyone's guess, if you don't have a way to monitor the downstream o2. It should stay relatively flat, as in not switching between high/low volts.

A broken ground wire at the o2 could cause the wire to randomly make the PCM see switching as it might vibrate from wind/what ever.

Mine has randomly come on, but only when it is idling at a light, or traveling at idle. Running a heavy percentage of E85 has kept it off for a while now. But the exhaust doesn't stink, smells like the cat is working, and it runs fine.

Eventually I will splice the connector from my old upstream to see if it stays off with 100% E10 before I buy a new downstream o2.
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2001 Neon ES MTX with '02 R/T PCM
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cdewalt1985
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Location: Carlisle, PA

Post by cdewalt1985 » Sat Jun 30, 2012 11:17 am

thanks for the reply
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