Replacing pre cat on 2002 Pathfinder

[stevecam1607]

First posting as new owner of 2002 Pathfinder 4WD with 97K miles. Getting P0420 code. Clearing code has it reappear within a week. Have changed coils, plugs, injectors, fuel filter, MAF, air filter, fuel pressure regulator, and downstream cat (obvious leak). Mechanic says both O2 sensors are tracking correctly. By process of elimination, apparently upstream pre cat is the culprit. How much of a pain is it to replace pre cat? Any way to confirm pre cat is the problem before I jump into this?

[PapaSmurf2k3]

Sounds like you've blown a lot of cash on things that you didn't need to replace.

Do you have smog checking where you are? If you get it checked and everything is in spec (or if you don't have smog checking), you could always install an anti-fouler to get the rear O2 sensor out of the direct path of the exhaust gases.

I've never replaced the pre-cat on the older pathfinder, but I'm sure there's a process outlining it in the factory service manual:
www.nicoclub.com/fsm

[atraudes]

You can look up the codes and what they mean in the FSM next time, to avoid shooting in the dark.

I had the same code when I bought mine, along with P0430 (the other side). I fixed it by replacing the catalytic converters. Cats don't typically just go bad by themselves, though. In my case I had a bad fuel pressure regulator which was causing the engine to run rich, which fouled up the cats.

In your case, since it's only on one side, the problem is likely confined to a single cylinder (unless the other is still on its way out). Make sure your regular maintenance is up to date, and address any codes being thrown, and any general issues like hard starting, rough idle, or black smoke.

That being said, you may have already fixed the root problem with all of the things you replaced!

Let us know how it works out!

[PapaSmurf2k3]

Have you tried clearing it and really beating the crap out of it? If the cat is fouled, it could have some crud on it, reducing its efficiency. Doing a few full throttle runs up to 60 or 70 should clear them out.

[stevecam1607]

Thanks for the responses. I initially had a miss which led to the initial tune-up items, miss is now gone. Only issue remaining is the P0420 code. I'll try some full-throttle runs and see if it has any effect.

[stevecam1607]

P0420 code still remains, although running fine otherwise. Fires up immediately. Combined MPG driving is 16-17 which sounds normal from other posting.
FSM and Chilton's does not really address details to replacing the pre-cat. Is it possible to replace pre-cat all from underneath access? Topside looks pretty congested. Will be a driveway project and current weather in Utah makes this a springtime project.
Appreciate all of the club site forum postings.

[atraudes]

16-17 is very decent. I track my mileage with an app and get 15 mpg, almost on the nose. I do probably 1 part highway to 2 parts street driving, and can have a heavy foot. The code and what it's detecting probably won't have any noticeable difference in starting or driving, though it's probably adjusting the timing and mixture to compensate.

You can replace them in the driveway and I think some folks here have, but I wouldn't recommend it or be able to offer any advice as to how to. I say that because I tried it myself and wasn't able to. The problem is that there are 6 or 8 nuts you need to remove to detach it from the exhaust headers, and half of those nuts are jammed up next to the firewall, and the cat blocks access to them. I couldn't even wrap my head around how to access them short of removing the headers, so I had a shop do it instead (and was glad to). Even if that hadn't been an issue, the bolts connecting it to the rest of the exhaust was crazy rusted over and seized.

Crawl underneath and scope it out for yourself, though, to see if it's something you could tackle. They're right in front of your face so it's pretty easy to see what would be done.