[Smacaroni]

I've got a CEL and P0420 code. I'm pretty sure my pre-cat is bad. The O2 readings on the upstream and downstream sensors are nearly identical except at idle. It's got 240,000 miles on it.
Simple enough, buy a new one, install it, take it for a drive and see the readings on the rear O2 are almost flat and the ones up front are peaks and valleys, right?
That was until I tried to take it apart. Everything is rusted solid and the fastener heads round off.
More than likely, I'm going to have to cut things apart.
I decided to stop before I got into a situation where I need to remove the manifold and drill/tap new holes.

I was looking online for a set of headers since the factory manifold looks terrible. I mean, it's an OK piece, for 1950, but there's got to be a better part available. I just can't seem to find it. I see stuff for V6 and 98-00, but nothing for the 2003.

Anyone have any good solutions, preferably one that will improve the exhaust flow.

 

[smj999smj]

Pacesetter used to make headers for the KA engine, so, try their site. I'm not sure it the 240SX headers will work on the Frontier. You may find some help in this thread:

http://www.clubfrontier.org/forums/f19/headers-ka24-auto-57908/

 

[Smacaroni]

Thanks, I read that thread before posting, but it actually gave me an idea, which I'll get to later.
The headers you can find are for a 1998-99 Frontier. From what I can gather based on the photos, that vehicle only has one cat and probably only has a single O2 sensor.
The 00+ (may be 01+) has a "pre-cat" and an O2 sensor on either side of it. My experience with other vehicles, the first O2 monitors fuel ratio and the second monitors emissions system efficiency. That's how mine knows there's something wrong.
What I could do is buy the headers in that thread, next install a bung in the collector for the first O2 sensor, then cut and install an ordinary weld-in cat after the collector followed by the second bung for another O2 sensor.
That is assuming that my 03 block is the exact same as the 98. If not, I'm kind of up the creek without a paddle.

Another issue is the headers you can find for the 98 are typically long runner style headers which are what you'd install on a high revving motor, not a low-RPM econobox.

There's one other idea that popped into my head, buy 240Z turbo headers which look like this:

Then fabricate an adapter to use the stock pre-cat and O2 sensor mounting to where the turbo should be. I've never heard of someone doing this, it was just a silly idea that I had while driving today and it's probably not a good idea either.

So, if you were me, what would you do? Right now, I'm leaning towards cutting the rusted fasteners off, removing the manifold and pre-cat as a complete unit, then putting it in a vice so I can put a torch on the fasteners to break them loose (I hope!) and if I can't break them loose, I can always drill them out and re-tap the holes followed by re-installing to stock. Especially since I already have the pre-cat and to return it, I need to mail it back to RockAuto.

 

[smj999smj]

I think you mean "240SX," not "240Z," which has a straight-6 engine. Anyway, earlier Frontiers had front and rear oxygen sensor; all OBD II compliant vehicles, which was mandated in the US in 1996, have front and rear oxygen sensors. I'm not sure how well the 240SX manifold will work; it should bolt up but I'm not sure about the fit with the steering column and everything else in that area. The earlier Frontier KA24DE exhaust manifold was very different compared to your Frontier's, likely due to the design of the catalytic converters.
The problem with headers is that you usually have to make some kind of fabrication unless it's a direct-fit replacement, shorty-style header. As you've already found out, header options are pretty slim when it comes to the KA engines, especially in trucks. Pacesetter is the only when I know off the top of my head that makes...or, "made"...them, but I'm pretty sure they were all long tube headers. As with most long-tube headers, you generally have to customize the exhaust on late model vehicles. The other issue is your state's inspection policies. The federal law considers removal or gutting of a catalytic converter as tampering with a federally controlled emission device, therefore, it is illegal. Some states have specific laws regarding catalytic converters, including California and New York State. For these reasons, most headers are labeled as "for off road use only." Where I live, you can get away with a bit more since they only do a safety check and no emissions testing. So, before you do whatever path you choose, keep in mind any issues you'll have when or if you bring your car to inspection.
As far as headers, unless you plan to do a lot more with your exhaust and intake, you are probably not going to get any noticeable gains in performance or mileage. If you were adding a low restriction exhaust, aftermarket CAI and performance cams, then installing headers would make sense. It takes a bit of work to get any noticeable power gains out of Nissan's "work horse" (as they proclaim) KA-series engines, which is why so many 240SX owners do SR20 swaps when they want to get serious with their vehicle's performance. Unless you are doing those other mods, you might be best served with the stock exhaust manifold and not deal with the headaches of trying to figure out what to do with the catalytic converter(s) and oxygen sensors, as well as the rest of the exhaust.
As far as removal, I would remove the manifold and pre-cat as an assembly. I'm pretty sure that's how the manual tells you to do it, anyway.

 

[Smacaroni]

My mistake, the photo I posted is from an eBay search for 240Z turbo header. I guess someone is in for a surprise if he orders one.
None the less, it sounds like I'll be pulling the manifold with the cat to replace it with the OER I bought. Just need an exhaust gasket first.
Guess I know what I'm doing next weekend.

Thanks for the help. I wanted to make sure that really was my best option.