Hey guys,

Trying to sell my 2003 QX4 and I need to get rid of the "Maintenance Engine Soon" light. Looks like its both a P0420 and P0430, "Catalyst System Efficiency Below Threshold" for both Banks 1 and 2.
That concerns me that its both sides at the same time. If it was just a bad O2 sensor, why would both trip? Does this there's actually something wrong going on? O2 Sensors are pricey for these cars, so I really don't wanna throw $160 at a car I'm trying to get rid of unless I know for certain its the issue.

What can I do to investigate further?
Anybody dealt with these codes together (or separately) that might know what's going on?

No misfires at all, and no other issues that I'm aware of, so nothing is out of the ordinary.

P0420/P0430 with no other codes almost always mean your cats are shot, not your sensors. There are other conditions that can cause it such as exhaust leaks in the vicinity of the rear O2's, but it's pretty unlikely you'd spring a leak there on both pipes. Other stuff upstream which can cause it will almost always throw other codes along with the dead-cat codes. The only thing you can try for mitigating it is O2 sensor spacers. If you're in Cali, you're pretty much screwed because spacers are a no-no.

O2 sensors are cheap as long as you're willing to get a universal and splice the connector on, just make sure it has the right number of wires.

How long have they been on? OBDII codes will self reset after 3 clear driving cycles in a row so if it's a one off, they should reset. On the other hand if they've been on for some time the cats probably are bad. Now they aren't cheap but make up for it by being hard to get to.

Oh great, not what I wanted to hear.
Should I buy new ones or can I take them from a junked QX4 at my local PullAPart? Any cheap sources for new cats? I do all my own work so as long as its feasible, I'll just do it myself.
Is it as easy as unbolting them, tossing the new one in, and good to go?

Maintenance Engine Light has been on for about a year now so...... yeah not good. But I don't drive it all that often as I've been wanting to rid of it for a while and have another car as a daily, but I've driven it definitely more than 3 times. Probably gets driven once a week on average.

Also, are the cats the same as a pathfinder? The same gen Pathfinder is much more prevalent at PullAPart, been over a month since any of the 4 in my area have gotten a QX4, but they get Pathfinders all the time

rckeeps2 wrote: ↑

Wed Aug 31, 2022 10:29 am

Also, are the cats the same as a pathfinder? The same gen Pathfinder is much more prevalent at PullAPart, been over a month since any of the 4 in my area have gotten a QX4, but they get Pathfinders all the time

Junkyards are forbidden by federal law from selling used cats, so that's not an option. If you're in Calif OBDII grade cats are required, which are much more expensive. If you're just under federal rules they can be had relatively inexpensively, but are typically short lived.

rckeeps2 wrote: ↑

Wed Aug 31, 2022 10:29 am

Also, are the cats the same as a pathfinder? The same gen Pathfinder is much more prevalent at PullAPart, been over a month since any of the 4 in my area have gotten a QX4, but they get Pathfinders all the time

'01~'04 Pathy uses the same cats.

VStar650CL wrote: ↑

Wed Aug 31, 2022 10:38 am

'01~'04 Pathy uses the same cats.

NICE!

Mike W. wrote: ↑

Wed Aug 31, 2022 10:35 am

Junkyards are forbidden by federal law from selling used cats, so that's not an option.

Nevermind That would explain why I couldn't find them listed on their pricing index.

Great, looks like I'll be dumping more money into my POS just to be able to sell it...

Any recommendations on retailers?
Im not in Cali, so that's good. Cheapest I see is about $150 each

or $200/set for ebay cats. might be what i go with

If you get really cheap ones, get a set of spacers along with them. Bargain basement cats are often weak right off the shelf, but a set of spacers will mitigate it and let you turn off the MIL. Since you're selling it, I'd do that preemptively.

Did you get this fixed? We had the same codes last year and had to replace the cats. One code kept coming up quickly and the other would come on after a couple hundred miles so we just did both. They were a little under $1k for both at RockAuto. Also, one of the downstream sensors wire broke off so I bought a cheap one on Amazon and it worked perfectly. The cats were easy to install with a long extension, rust was not an issue. This is for California.

btw, my son is selling his PF too, after 6 good years.