Posted by: @jisoo

What has been replaced so far

what about air filter?

Posted by: @jisoo

Will my catalytic converter be bad?

see the FAQ under P0420

An inefficient catalytic converter doesn't affect the air:fuel mixture. However, a melted cat will restrict the exhaust gases exiting the combustion chambers and with incomplete exhaust gas evacuation there's less volume in the cylinders ((and less vacuum for the components that depend upon it)) to be filled with O2 on the next intake stroke of the engine.

The engine breathes. If you want to understand how this happens you can inhale fully and then exhale for 2 seconds through your mouth and then inhale fully again.

See how you took in a nice full breath?

Now, Inhale fully and place your hand very tightly over your mouth and try to exhale fully through your mouth for 2 seconds. Remove your hand and try to inhale fully again.

See how you couldn't inhale the same volume of air as the 1st time because your lungs still had a lot of carbon dioxide in them because you couldn't empty your lungs with your hand obstructing your ability to exhale?

That's how your engine feels with a melted/clogged cat blocking its ability to exhale.

It has waste gasses stuck in its lungs (cylinders) too and can't fully inhale fresh O2.

If you kept that up, pretty soon you'd have trouble running very fast. So does a car. 

In an engine cylinder, any exhaust gases that remain are non-combustible (because they are a waste product of the previous combustion stroke of the cylinder.) So on the next intake stroke less O2 will enter the cylinder. And there will be less O2 in the exhaust and the upstream O2 sensor will see that. The computer will interpret that as a Rich mixture and start cutting down the fuel injector pulse durations (Negative Fuel Trims). 

You can see how to test for a melted/obstructed catalytic converter by watching this video from Scotty:

https://www.youtube.com/watch?v=qJTmddbOxMk

I'm impressed that you went to the tailpipe for clues and you noticed it was black. Stick your finger in there to see if it's sooty black (Rich) or oily black (Burning oil) 

Either of these conditions will kill a catalytic converter. They'll also cause the cat to work inefficiently on the way out. If you have black sooty deposits in the tailpipe you have a rich condition and you should troubleshoot that 1st. You can probably resolve that one. With black oily you can check the PCV valve but beyond that you're hunting more serious issues.

So that's that but one more thing. You're suspecting a bad cat but you didn't mention any trouble codes that would point you in that direction. 

It seems we don't discuss catalytic converters on the forum in much detail and maybe we should to remove the confusion. The guys (or the gals) typically come here complaining of a P0420 or P0430 code and want to know if they need to replace their cat(s). (I know that you didn't but don't try to stop me now, I'm on a roll) It isn't that simple for several reasons. 

The first reason is that those 2 codes are "Catalyst System Efficiency Below Threshold".

The catalytic converter works efficiently at Lamba 1. Air:Fuel Stoichiometry.  (air:fuel mixture at 14.7:1). Deviate from this either way and the cat efficiency decreases.

The Catalytic converter has 1 job. To render poisonous exhaust gases harmless.

But like I said, it needs to start with a 14.7:1 fuel mixture being ignited for it to work correctly. They call that "feed gasses" to the cat in the automotive lingo.

It may be helpful to some forum members to understand the simple function of the catalytic converter.

These days we call them 3 Way catalytic converters because unlike the earlier cats, the newer ones convert all 3 poisonous exhaust gases in a single cat unit. NOx (oxides of Nitrogen ) are converted into harmless Nitrogen and Oxygen).

HC (Hydrocarbons)  into harmless O2 and H2O.

CO (Carbon Monoxide) is converted into harmless CO2 (Carbon Dioxide)  ((I know. global warming from CO2 emissions but it isn't poisonous in reasonable levels.  Heck, people, animals, and even trees release it)). The upper atmosphere doesn't seem to care for it. That's a different discussion.

So looking at this graphic we can see how the catalytic converter does this and it converts the gases in 2 stages using 2 different catalysts. 

Let's look at the precious metals attached to the ceramic substrates and see what they do and when.

The "big picture" is that the cat converts the poisonous NOx, CO, and HC, into harmless N2, CO2, and H2O.

The first stage has the ceramic substrate coated in Platinum and Rhodium and it converts the NOx into N2 and O2 while having no effect on the CO and HC gasses.

But for the next stage of the operation the precious metals in the second ceramic substrate coated in Platinum and Palladium needs that O2 that was stripped off of the NOx to convert the CO (carbon monoxide) and the HC (hydrocarbons) into  CO2 and H20.

The catalytic converter is able to store the O2 and should never become depleted of it because it can't work without it.

And that takes us back to why we should resolve any air:fuel mixture issues or misfiring cylinders before condemning the catalytic converter 

Possible vacuum leak in the intake system forcing the computer to provide overly rich mixture to compensate. Check all connections starting at the air filter box to the intake manifold.