HI, I’ve been getting P420 on my Hyundai Elantra 2004 (about 130K miles). I got scanned data below. Would you tell me if any number number is way off the normal range?
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
G=GREEN CHECKMARK
S=STOP SIGN
X=X MARK
MIS G
FUE G
COM G
CAT G
EVAP X
O2S X
HRT G
I believe this is the short-term fuel trim set based on the downstream O2 sensor.
n/a could mean there is a fault in sensor or wiring. but others here will know better
If it’s a 4cyl. There is no Bank 2 sensor. So I guess N/A could be expected. If people can’t say the engine in their car, even if there was only one option, I ain’t wasting my time finding out.
The only engine available in the US was the 2.0L I-4.
The OP might not be in the US .
All engines were I-4 regardless of where sold.
Edmunds lists the Sonata of some years having a V6 .
The wiki shows 4 I-4s in 2004 worldwide.
I don’t see anything way off. But the 5th line, long term fuel trim of 6.3 %, that is on the high side, probably pointing to some problem brewing at least. It means the drivetrain computer is being forced to inject 6.3% more fuel to meet the O2 sensor fuel mixture requirement, than it would otherwise based only on the intake airflow. Pretty good chance air is leaking into the engine somewhere, bypassing the airflow sensor. Check the intake system air boot, see any splits? Ask your shop to check the pcv system, problems could allow an air leak. Exhaust leaks could cause it too.
p0420 means the signals from the pre- and post-cat o2 sensors are not matching like they would if the cat was working properly. The best way to diagnose that is for a shop to monitor those two signals side by side using a scan tool. Could be a faulty cat, or could be whatever’s causing the higher than expected fuel trim. O2 sensor failure possible, but unlikely.
My guess, the extra gas has been injected over a long enough time that an incorrect fuel mixture has damaged the cat, and you’ll have to fix what’s causing the fuel trim problem along with replacing the cat.
Have someone connect a scanner that can display real-time data and monitor the up-stream and down-stream O2 sensors,
If the down stream O2 sensor matches the up-stream O2 sensor, the vehicle needs a new catalytic converter.
Tester
Thank you, George_San_Jose and Tester, for the reply. I checked the intake system air boot. Pretty good. No crack or split. All clamped down tight. I’m going to check the O2 sensor before I check other things since o2 sensor seems to be the easiest and cheapest part to replace. One thing though, you said p420 means the pre and post o2 are NOT matching. In a normal situation, if I didn’t have the P420 code and If my CAT and the O2 sensors were all good, shouldn’t the pre and post o2 sensor volt readings be different? As “Tester” has said if the pre and post O2 readings were matching, that might mean the cat is not doing its job. Am I correct?
So, you’re going to replace O2 sensors, even tho there’s no O2 sensor codes? 
Tester
Since George_San said O2 sensor failure is possible and I got the red x mark for the O2S, I thought I give that a try. Tester, you suggested graphing the live data of pre and post O2 voltages to see if they match. As I posted originally, O2B1S1 = 0.490 & O2B1S2(v) = 0.035. Doesn’t that mean they do not match?
I am going to get flagged but I don’t care. George is the resident guesser for this Forum .
In his defense, a lot of mechanical work is guessing and figuring the problem out. isn’t it? especially on a forum that you can not see the vehicle.
I was expecting some P420 Marijuanna joke!
Yes, but the o2 sensor’s entire signal is used for the comparison (voltage vs time). The voltages might be the same at certain instants of time. But the cat’s functionality can’t be determined by one voltage at one time. You need a way to chart the time history of both signals. By “match” I didn’t mean the voltages have to be exactly the same at all instances of time. I meant the relationship between the two voltage-vs-time signals has to match the specification.
No George.
That’s what you typed.
And that’s misinformation!
PERIOD!
Tester
What do you other folks think? Did I provide complete misinformation? Or did I provide something mostly useful to the OP?
George try something new . If you are not sure of what you are posting then just do not post .
You don’t have to post to every question that comes here .