P0420 code

[jaworski88]

Can a misfire on one cylinder affect both catalytic converters?

To make a long story short:

I ask because I had a bad ign coil on cyl #4 (bank 2) that caused a misfire which ruined my cat on bank 2. After fixing the misfire, I got P0430, which is catalyst bank 2 below threshold. I replaced the cat on bank 2 and the code was cleared. After about 600 miles driving, now I get a P0420, catalyst bank 1 below threshold. I have 163,500 miles on my 2002 QX4.

Could these two cat failures be related due to the misfire on cyl 4? Or just a coincidence?
How long/miles do these converters last?

thank you in advance!

[nickelghandi]

How long did you drive on that bad coil? If not burned gas was flooding into the cat it could have killed it, but that is usually after hundreds or even thousands of miles and by that time you would wash out your piston rings anyway.

Try clearing the code and continue driving. If it comes back, then you might be looking at getting a new cat. They usually last a very long time, even the life of the vehicle, unless subjected to a lot of fuel or oil contamination.

The fact that one happened right after the other is strange. It could be another issue entirely, or just time to replace both due to wear.

[jaworski88]

Nickelghandi, thanks for your response.

I bought the car recently with a known faulty ign coil on cyl #4 (bank 2), and I gathered that the owner drove it for a month or two while misfiring. So that explains why the cat on the bank 2 side (driver side) was damaged, but I didn't think the cyl #4 misfire would affect the cat on the bank 1 side. Is it possible for some of that unburned fuel from cyl #4 (bank 2) to reach the cat on the bank 1 side and thus ruin that cat as well? I say no, but thought i'd ask you all here who are a lot more knowledgeable about this car than I am. So am I wrong, or was it just a coincidence that the bank 1 cat went out not long after replacing the bank 2 cat? I am just trying to understand this car better. I appreciate your input...

[Mike W.]

Have you replaced the post cat O2 sensors? They typically last much longer than pre cat sensors, but not always. Not quite cheap, but much cheaper than cat and a ton easier to replace.

[jaworski88]

Mike W.
No, I have not replaced any of the O2 sensors. I suppose I should test those before replacing the cat on bank 1 this time. Thanks!

[macgiver]

mac, my 93 J30 264kmi ,separate dual exhaust , original cats , L. one has a very small "chip" floating around -fully functional though , chip usually settles 10 min. after warmup.But yeh test the O2's, they're fraction of the $$$$$ of a cat.

[nickelghandi]

Oh yes, definitely check the O2 sensors.

I don't want to say it would never happen, but it is pretty much impossible for Bank 1 to be contaminated by something wrong in Bank 2. The two pipes don't join until way after the cat and AFAIK there is no connection between the two other than that. I am betting O2 sensors, something else wrong, or just coincidence. If the PO replaced the cats once already and used cheap parts then they could be failing due to low build quality.

Cats can fail if you run it too full of oil as well. Does it smoke? I once bought a smoky, leaky Honda that had been overfilled killing the cat and causing all sorts of engine codes and misfiring. I had to pull the intake manifold to clean it (like a 40 minute job on a Honda lol). I replaced the leaky seals, the exhaust from the manifold back (including a new cat) and spark plugs and it cured it all. Sold it at a nice profit too.

You say you bought the car recently. Have you changed the oil yet? Examine it closely for sludge and make sure the correct volume comes out. A good trick is to dump the old oil into the new empty jug if you buy it in the jug. My dipstick always reads low on this vehicle so the only way to tell if I am using oil or have too much is to change it. I use about 2 quarts between changes which I do every 3500 miles, so I add a quart at 1750 miles.

[jaworski88]

Hey, Folks! Thanks for you inputs.
nickelghandi: "... it's nearly impossible for bank 2 to contaminate bank 1..." thanks for the confirmation, because that's what I was thinking too.

Also, I changed the oil right away when I got it, so it is at the right level. However, your comment about the oil made me wonder about something else that I think may be the actual source of my problem. The PO said a shop once told her that it has a valve cover gasket leak, and quoted her $3000 to replace both gaskets. I've confirmed it, it is leaking, but not so much that it's dripping all over my driveway yet. So I think that VCG leak is why I might be getting that P0420 code on bank 1. That is, the oil leaking from the bad VCG may be finding its way into the exhaust on bank 1, and thus fouling it up? Is that possible? If so, then I should replace those gaskets before doing anything else. What do you think?

Thanks again!

[Mike W.]

That is, the oil leaking from the bad VCG may be finding its way into the exhaust on bank 1, and thus fouling it up? Is that possible?

Thanks again!

No, that is not possible. A VCG leak is external, it will find it's way to the ground if bad enough, but not into the exhaust or anywhere else.

[ianh]

I have this as well, 155,000 miles but only one bank.
Any update, was it the O2 ?
Thanks

[Rockwood]

I had the 420 code -replaced the downstream (rear) O2 sensor and that took care of it. That was 4 years ago I think - no codes since!