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unsaint32
New User
Jan 29, 2023, 6:25 PM
Post #1 of 4
(1524 views)
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P420 check engine code
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My 2004 Hyundai Elantra gets P420 check engine light that goes away when the fuel tank is at about 80% then comes back on when it drops to about 60% and stays on until I fill the tank and the level goes down to about 80% again. Can you give me some ideas as to where I should start looking? Here is my scanner readings.
FUEL SYS1 CL
LOAD_PCT 13.7
ECT –
SHRTFT 0.8
LONGFT 6.3
RPM 767
SPARKADV 4.5
IAT(C) 9
MAF(g/s) 2.29
TP(%) 6.7
O2SLOC B1S12–B2S
O2B1S1 0.490
SHRTFTB1S(%) 3.1
O2B1S2(v) 0.035
SHRTFTB2S(%) N/A
OBSDUP OBDII
MIS - GREEN CHECKMARK
FUE - GREEN CHECKMARK
COM - GREEN CHECKMARK
CAT - GREEN CHECKMARK
EVAP - X MARK
O2S - X MARK
HRT - GREEN CHECKMARK
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Hammer Time
Ultimate Carjunky
/ Moderator
Jan 29, 2023, 7:00 PM
Post #2 of 4
(1521 views)
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Re: P420 check engine code
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That data means nothing for this code.
Neither does the fuel level.
95% of the time this is just a bad cat.
About the only other thing that can trigger this code is an air leak in the exhaust ahead of the downstream sensor.
This code doesn't set by accident. In order to set this code the front and rear 0/2 sensors have to mirror each other.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
(This post was edited by Hammer Time on Jan 29, 2023, 7:02 PM)
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Jan 29, 2023, 11:42 PM
Post #3 of 4
(1491 views)
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Re: P420 check engine code
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Hammer is probably spot on.
I'll add for the 5% what else to check it's pushing 20 years old! Is fuel tank consuming the fuel but air thru evap syst. not replacing it as it's a vaccum in there. It would lose power and come back it cap was off.
To do that would take plugged evap system, wrong or bad cap any would make it run so lousy that you didn't mention you'd notice.
Everything about air intake thru all exhaust parts in order, items all plugged in then underside the same for exhaust. A gas weed whacker would need that much too it's the way fuel is useful in an engine,
Tom
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Sidom
Veteran
/ Moderator
Jan 29, 2023, 11:51 PM
Post #4 of 4
(1491 views)
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Re: P420 check engine code
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As HT said a 420 is Almost always a bad cat There are few other things that can set a 420 But usually you will get a code As well for the bad part Or failure causing the 420.
Since the code is coming and going, It sounds like the cat is just at the beginning stages of failure. Sometimes it passes the test and other times it fails. The Computer compares the upstream O2 reading against the downstream O2 reading to test the converter. Using a scan tool to check a converter, you have to look at graphed readings For it to make any sense. Looking at numbers is too hard to tell because they are always changing quickly. When you graph the numbers It is really obvious when you have a bad converter. The readings look identical Between the upstream and downstream O2s are identical.
An old smog trick was to run a can of sea foam in a full tank of premium gas. That would usually clean the cat enough to clear the 420 long enough To run the smog and it to pass. But the 420 always comes back eventually and the cat needs to be replaced
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