[peacew]

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

 

[chandler48]

P420 is a bank 1 catalytic converter error code. Not sure why it is reacting as you indicate with filling the tank, then depleting it a percentage. Have you checked your O2 sensors ahead of the cat

 

[Matt1963]

Or your gas cap?

If it's running good it could be the downstream.

That code can be many things because it just means that cat is running under efficiency. A 5% percent efficiency can light up the code.

There is a product called Cataclean you can put in your gas tank but I've never used it.

 

[Akcabin]

Maybe that 420 code means go smoke some pot n chill out because fixing it is a oofda moment.