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Old 01-07-2018, 02:41 PM   #1
aarons mom
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Po 420

I have cat code PO 420 on my 99 accord EX, 4cyl. I have a few questions. I had new pipes and muffler braised on. The muffler shop said the cat looked good. Should I start with trying a new O2 sensor? Could it have heat damage? Also, I need to know if I have 2, O2 sensors and do I change both? I think this is my "family" 3PL4 on the hood, F23A4 on my motor.
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Old 01-17-2018, 07:17 PM   #2
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Re: Po 420

bleh... p0420 codes. finding the source is a pain in the tuchas.

i wouldn't go to the expense of summarily changing out the O2 sensor(s) just yet. that's not to say a wonky O2 sensor isn't the culprit. (if you have a set of known-good OEM O2 sensors handy, go ahead and swap the rear one first, clear the ECU, and evaluate--if the code returns, replace the front, clear, and evaluate--if the code returns, the source is elsewhere.) same goes for the cat. you definitely don't want to replace that hurty-expensive part unnecessarily.

to be completely blunt, p0420 codes are troublesome for the shade-tree mechanic to t'shoot. there are nearly a dozen things that can cause that code, none of them pleasant. i humbly suggest you schedule some quality time for your Accord with a local shop that you trust, preferably one that is Honda-centric (Google it), and purchase a diagnostic.

proper t'shooting typically requires use of a professional diagnostic scanner to capture live engine data during back-pressure and vacuum tests. explain to the shop that you just want the diagnostic--not any remedial work. if the report comes back as "faulty front O2 sensor", then you can most likely source and install a new OEM sensor yourself. if you get really lucky, maybe it'll just be a leaky compression gasket at the exhaust manifold.

if you wanna try t-shooting the code yourself, you may find this thread a useful resource.

Last edited by lothian; 01-17-2018 at 10:06 PM.
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Old 01-17-2018, 08:18 PM   #3
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Re: Po 420

thanks for your info. i probably should have told you that
i had a tank of bad gas that triggered a 1399FLASHING CEL, then the cat code popped up. Since, I have determined that the ECM can not tell the difference between unburnt fuel and water...I have replaced every component tank side except fuel pump, done timing belts and water pump and now I have replaced pipes and muffler exhaust side except O2 sensors and cat.My middle name is 1456. I do have a code scanner and I do have a very trustworthy mechanic. I am running much better, no spits and sputters since I ran the bad gas out (w/dry gas), gonna clear my evap and cat code and see what happens...
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Old 01-17-2018, 08:30 PM   #4
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Re: Po 420

interesting.

i wonder, then, if poor fuel quality caused--somehow--the ECU to induce a lean condition. i believe a lean condition raises combustion temperatures which, in turn, raises exhaust temperatures in excess of the operational efficiency temp range of the cat (900F-1400F) which, in turn, can trigger a p0420 DTC.

but still... bad gas..? is that really a thing? nevertheless you want to be genuinely prudent in this regard, so you might consider visiting pure-gas.org and locate a source of ethanol-free fuel for your Accord. it'll be a pricey but an effective method to both purge and prime your fuel system. the j-series engines are indifferent performance-wise to the octane value of fuel; however, they do perform very nicely on the non-ethanol stuff.

i'm suspicious bad fuel is really the cause of your p0420, however.

Last edited by lothian; 01-17-2018 at 10:11 PM.
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Old 01-17-2018, 08:52 PM   #5
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Re: Po 420

I do not drive often. It has taken over a month for me to run the bad gas out. I had to stop to add dry gas 3X before I quit having problems. I am good now. i just have not cleared the codes and run it. Fresh gas. I do buy non ethanol for all my small engines...the only way to go and I do use mid grade in the car. I just bought gas on the fly one day, not my normal station. Since, my mechanic told me that I was #2 in the shop w/ bad gas from the same station. At first i was freaked and thought it was my head gasket, but now that I have all kinds of power back and no flashing cel (and no milky oil) I've abandoned that one. When i had the moisture problem I was smoking like crazy, white smoke, it did not smell like coolant or oil. I figure it was the dry gas/water evaporating in the cat. this has stopped. So, my plan is, clear codes, drive, if codes come back, change downstream O2 first. then upstream, then cat. I'm going to throw in a new PCV valve, I'm sure it was not done when the belts were done. OH, yes, I was running very lean with the moisture in the gas.
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