[Monashee]

I've got all the parts (cats, o2 sensors, gaskets, mid-pipe, fabbed y-pipe, hardware) and all the tools ready. Few questions before I start though, since I'll be doing it at my friend's place 2-hrs from the nearest dealership.

Electrical...
How to "reset" ECU? Disconnect battery, wait a min, touch cables together for a minute?
Then disconnect o2 sensor connectors. Anything else?

Exhaust Removal/Install Order...
Was planning the removal starting at the y-pipe going forwards, then front-to-back for the install. Sound about right? Easier way?
I'll just break the old o2 sensors off since they're getting replaced anyway (have correct sockets though).

Lock-washers necessary on pipe flanges? I'm using 1/2" bolts and even so there's still enough play in the holes, but couldn't find 1/2" flange nuts so I only have regular nuts and flat washers. Add lock-washers?

Cat-Manifold flange... What's the chance of me breaking the single stud on each side? That would suck.

Heat Shields...
I'm sure the bolts will break off, and I won't be able to tack-weld them back on at the time. I know they can be removed without mangling cutting them (yes, even the pass. side one). Will I be able to get them back in there afterwards, with the cats still in place? Or just run without?

Replace/Service other things while in there with access?
Any other tips? Things I haven't thought about?

 

[Monashee]

Got it done today. 5-hrs start to finish inc lunch and breaks. Exhaust work is never fun. Simple work but tedious... cramped quarters, tight access, rusted bolts, gravel driveway (plywood helped).

Reset the ECU via "The Easy Method" described here: http://www.clubfrontier.org/forums/f8/need-reset-ecu-79125/#post1094191
Seemed to work (SES light went off). Then disconnected battery and touched cables for 30 seconds.
O2 sensor wire harness clips are a PITA (like most electrical clips on all vehicles) but eventually got them apart.

Not a chance removing bolts on each end of secondary cats; far too caked/rusted/seized (cleaned with chisel & wire brush, PB blastered, applied heat... snapped the first one... pretty much a waste of time). Switched methods: cut pipes with angle grinder; was quick and easy.
Manifold heat shields each had only 1 remaining bolt o begin with. Driver's side bolt snapped off easily and was then able to maneuver shield far enough up out of the way. Couldn't access remaining passenger side bolt (lower bolt) & didn't want to waste more time as it surely would have snapped anyway, so I cut a 2" chunk off the tail end of the shield with the angle grinder which allowed just enough access for the nuts.
The 3 manifold-to-cat flange nuts on each side were surprisingly not rusted at all and very easy to undo (except for the painfully tight access... ratcheting flat wrench was the ticket).
Install was much easier and about 1/2 the time of removal. Installed the O2 sensors prior to putting the cats in (definitely easier this way). Reused OEM flare nuts at manifold-cat junction. New 1/2" bolts for both ends of secondary cats.

Parts used are seen in attached diagram. 4 cats, "front pipe", and gaskets. All fit great. Old cats were not clogged up with any debris at all. O2 sensors were overdue. Truck seems to have original power resorted (only noticeable when climbing hills). Sound and volume are the same as factory as far as I can tell.
Will upload some pics of the old cats tomorrow & the newly installed system later this week.

 

[Monashee]

I'm in BC Canada (most expensive auto parts/service in North America).
Parts are cheaper as you head east in Canada, and much cheaper (often 1/2 price) down in the States. That being said, I think I got a great deal on everything...

One primary from Amazon.ca on sale for $132 free shipping + $16 tax = $148 cdn
Other primary on sale at RockAuto for $170 cdn + $16 shipping + $22 tax = $208 cdn

Both secondaries from RockAuto for $160 x2 + $33 shipping + $42 tax = $395 cdn

They all came with the proper gaskets.
Picked up hardware at the local fastener store... probably $20 cdn.

3 oxygen sensors from RockAuto... two on sale for $45 cdn each & one regular price of $88 cdn. So $178 cdn + $20 shipping + $24 tax = $222 cdn
4th oxygen sensor from ClubPlug.ca for $111 cdn total inc. tax & shipping.

I then had mail-in rebates for the Walker cats (got $160 back)
So $900 cdn total in parts -$160 rebates = $740 cdn ($568 usd) + tax & shipping. That's for 4 cats, 4 oxygen sensors, gaskets & hardware.

Even going with the same parts I ordered (Walker & NTK) but from stores in Canada, would have upped the cost by about 40% and would have needed to be ordered-in anyway.
The quote for a single primary Nissan cat was over $1000 cdn...

 

[Monashee]

Pics of the old cats

Had about 143,000 miles (230,000 kms) on them.
Insides still "look okay"... intact and not clogged ... but had lost their efficiency.
O2 sensors had same mileage.

1) all 4 cats & mid-pipe
2) inside of primary
3) inside of secondary