Sorry, @George_San_Jose1
I think the information you provided is very likely leading OP down the wrong path
btw . . . I wasn’t planning on chiming in at all, until you asked for our opinions 
1 Like
That’s not possible without having the MIL on with a mixture too rich code, or a completely non function O2 sensor, in the past and continuing to drive it that way.
That’s what I thought, at least after the catalytic converter is warmed up and working. It should produce more Voltage due to the lack of oxygen in the gas it is measuring right? The article I linked just says that the post converter sensor should remain stable as the pre one changes.
Does the x mean that the readiness monitor is not complete? This happens after someone clears the codes. After the required driving cycle it will change to pass or fail.
The following article explains how a lazy O2 sensor can trigger a false P0420 code. It explains how to read the sensor 1 compared to sensor 2 values. George_San_Jose1 isn’t wrong to suspect this then. Testing O2 Sensor
1 Like
Thanks for posting. Do you concur w/@Tester that what I posted is “misinformation”? Or are you saying what I posted is correct, but may not be pertinent to the OP’s problem? I’ll grant the latter might be the case, but I don’t think what I posted was misinformation. As you know I’m just a simpleton diyer with no OBD II vehicles, so no personal OBD II diagnostic experience. In the way of lighting a candle rather than complaining about the darkness however, suggest you inform the OP what you believe to be the correct path toward a diagnosis?
I think you gave OP false hope when you started talking about the oxygen sensors
And I also think mentioning the fuel trims wasn’t particularly helpful, as that screen shot doesn’t show anything very noteworthy, imo
Bingo , we have a winner
This is what I said: “O2 sensor failure possible, but unlikely.”
Well, it’s been my experience that in those situations, people will often focus on the unlikely possibility they only need oxygen sensors, versus the much greater likelihood they need a cat
It’s like the folks that come here with transmission issues hoping someone will advise them “it’s only a solenoid”.
1 Like
5 Likes
he DTC code P420 indicates that the catalytic converter efficiency is below threshold. This can be caused by a number of factors, including a malfunctioning oxygen sensor, a fuel system issue, or a problem with the catalytic converter itself. Here is a guide on “How to Fix Hyundai P420”
1 Like
Everyone, thank you so much for inputs. I’ve learned a lot. One thing I want to add is that the check engine light goes away when the gas tank is between about 90% and 60%. I would think if the cat was bad, the check engine light wouldn’t go away like that. Sorry I didn’t add this info before. I don’t drive a lot. I only fill the tank about once a month. And I totally forgot about that.
That is indeed odd behavior for a failed cat problem. hmmm … only thing I can think of is your battery is getting low voltage during the longtime car sits unused, and that erases the diagnostic codes. New codes and check engine light may not re-appear until cat test is next run, and test may only be run when gas tank level is between certain limits. Some OBDII diagnostic tests are like that, require gas tank level to be at a pre-defined fill level first.
Engine performance problems which vary with gas tank level can be caused by improper fuel pressure regulation as well.