I'll chime in with my recent issues.
I have a 2011 QX56, with 84,5xx miles. I have used premium fuel for every fill-up (sometimes 87 on long roadtrips) and I've changed the oil every 3500-4500 miles. The Service Engine light came on about 500 miles ago and I had my local trusted mechanic read the code and reset it. It was code P0420, which I googled and thankfully found a bunch of info on this forum. I was hoping the code was thrown because my wife didn't tighten the gas cap enough when she filled up, but unfortunately that wasn't it. A few days later the SES indicator lit up again.
So I took it to my local Infiniti dealer (Atlantic in JAX, FL). The service adviser seemed very familiar with the issue. She said since I was out of warranty, they couldn't do anything but provide a quote, but she suggested I call Infiniti Consumer Affairs to see if they could help out. I was expecting an estimate of over $3k, but they did me one better - over $4k plus tax! *UGH* This was for both converters.
I got home and put a call into Infiniti Consumer Affairs. They took my information and promised a callback by end of next day. Next day arrives and I get a call from the regional manager. She proceeds to read the cookie cutter response of "I'm sorry, but we cannot be of assistance, as the parameters of your warranty have expired". I made it clear that I wasn't happy, but I was expecting that response from all the google results with people having the same issue.
The rest of the afternoon was spent making phone calls. Jacksonville recently got a second Infiniti dealership (Orange Park), so I called them to get a quote. Turns out the old service manager runs this place and he was very helpful with information and a quote. His quote was roughly $3250 - WAY less than the other dealer. He was miffed how the other dealer came up with that high of number, but I didn't get the parts/labor breakdown from the dealer. Then I called a longstanding muffler shop here in town (Muffler City), and they said they've started to see a lot of these and did two QX56 catalytic converter swaps last week. For $1,250, they'll weld the catalytic converters off the existing manifolds and reinstall new ones with a 5yr/50K warranty. So I guess that saves big on the labor of having to breakdown the engine bay to replace the entire manifold.
I'm not sure what I'm going to do, but I'm leaning towards the cheaper welded option. Any dealer work from here on out will be going to Orange Park Infiniti though - just based on the information I gleaned today!
Later,
pjdavep