Before the transmission replacement the car would only reliably go into MM when cold; as soon as the fluid got a bit warm, it would stick in MM when putting it in regular D,, change to D after a few seconds, then stay stuck in D and refuse to go back to MM. A restart of the car would fix it, as well as starting the car and shifting to MM as soon as the revs fell (having to reverse or park almost always meant I couldn't get back to MM). Ended up finding aluminum shavings in the pan, only thing aluminum was the casing itself, soooo yeah. You can see what state the transmission was in.

THAT BEING SAID, I still had the shifting issue the same day I got the brand new transmission; downshift would sometimes not work, upshift always would (and about a year down the line the car once or twice did the same "stay in MM when shifting to drive, freeze, get stuck in drive). I swapped the entire shifter assembly for one I got from an online source, can't remember the place; don't think it was actually a new one, as it was only $300 instead of the dealer quote of $500. Regardless, it didn't seem to fix the issue any that I noticed. I have noticed a few things though;

1. Transmission is more likely to downshift in MM when cold OR when driving aggressively and moving a lot of air into the radiator. Just dropped to around 35-40 degrees from 50-60 a few weeks ago, and it's working almost everytime. We know that heat raises electrical resistance; it's possible that rising temperatures affect the solenoids in the valve body.
2. In the rare (twice ever, both same day) time when the car gets stuck in MM (after the new transmission), it doesn't change the screen to D and locks out MM; if I requested a downshift that wasn't given, the car would stay in the same gear for a few seconds and then downshift as many times as possible with the rev limiter, almost like it was receiving repeated commands (I was able to stop it by upshifting). If I waited until the car stopped downshifting, I could swap back and forth between D and MM no problem. If I tried to slap it into D while it was going nuts, it would lock out MM.
3. Cleaning every ground in the car helped a little bit, but not so much I could notice it. Swapping a failing camshaft sensor with a revised OEM one did seem to help a bit. I don't know what parameters the TCM looks for when requesting a shift, but it's possible it reads data off the camshaft/crankshaft sensors to determine if a downshift will blow up the engine or not. If it can't get a good reading, it may deny it.

In addition, my gauge cluster went bad when I had to dismantle it to fix a fuel gauge needle that went backwards to the other side of the peg when my battery died in a rainstorm (that was a fun one). Overall my left turn signal is either faint or doesn't work, my parking brake light doesn't work, my fuel gauge does whatever the hell it wants and part of the needle/backlighting of the speedometer doesn't work; HOWEVER all lights come on/mostly come on if I push the gauge cowl around a bit or raise the wheel (raising only helps the turn signal, and even then only sometimes), and the fuel gauge occasionally (i.e. once a month) shows the correct level for a few minutes (I don't think it was bad when I first got the car, but wouldn't be surprised if it was, it's a known problem). I'll eventually swap a new one in or fix it, but it's not necessary for getting to work, so yeah.

I find it unlikely that the gauge/combination meter can affect MM, but you never know, wiring can be complex. I've checked all the wires below the shifter, but I've found nothing out of place so far; my guess is that the processor for the downshift is going out. Speaking of which, the manual mode switch is part of the assembly; if you slide the shifter veeeeery gently between MM and D, you'll hear the tiny click of it actuating. It's a tiny switch that sends an electrical signal, so if it's bad it could cause issues; I find that unlikely, though. I plan to eventually install Works Bell paddle shifters, which will answer my question if the OEM down/upshift processors (the little clicks you hear while in MM and shifting) are my problem, as the WB kit bypasses those via a plug-in harness. '

Check your fluid first, and top it off with S-Matic if low; low fluid is the number one cause of problems like this. Do a drain and fill if need be, that never hurts. If you wanna get your hands a little more dirty, get the transmission pan dropped and checked, just to see if the transmission is dying or not (mine looked like a glitter bomb had gone off inside). If that all checked out, consider a new valve body, as those solenoids within are vital to MM (personally, I'd take the opportunity to install an upgraded one/one with a Transgo kit; from what I've seen, it's amazing. Should also reduce temps via faster shifts!). If THAT doesn't do it, it's something in the electrical line, which will be a royal bitch to diagnose (recently booked a customer car that replaced an engine harness due to rodents...36 hours of labor. Yikes).

Make sure to get back to us, I'm super curious how your case turns out. This is my last real drivetrain performance gremlin; once it's solved I can fully devote myself to making my damn interior stopping rattling/squeaking/tapping.

 

Sorry, not any help with this comment, just a question or two... Infiniti suggested to replace the shifter assembly, did so, it didn't fix it, and they charge you ? Same with the valve body. How does that work ? It's a different story if you told them "replace part 'x'" and they do so and it doesn't solve the issue but here these fixes were solutions they came up with !

 

I had both the shifter assmebly and the Valve body done at my Transsmisison shop.. But the car was diagnosed at Infiniti and they reccommened these solutions. You can see why I am reluctant to fix the combination meter, after blowing cash on thier reccommendations.
After I went back and said that these suggested fixes didn't work, they spent a half the day (or a few hours) looking the matter, at not charge, mechanics said that Nissan Techs on the phone would say the next logical fix would be the combintaion meter... uhhh! That's the last thing i want to do..

I'm checking my fluid and tranny condition next week, but everyone says it's fine where I go. I will update everyone after that!

 

I certainly haven't read all of that but the diagnosis from the dealership said something about a persistent pending MM switch code, but the switch tested fine. I'm sure you've already checked around but it's got to be something related to the signal of that switch... Either the wiring, or a computer misinterpreting it. I'd be interested to see what it would look like if an LED test light were wired up to the MM switch, and observed while driving. You could also hook up that setup to each switch in the shifter assembly to confirm that each is behaving properly. Maybe you could take the car to an automotive electrical specialist instead of an indy or dealership, and have them check the dynamic continuity of every single transmission wire and the correct signal output from every related computer.

It doesn't seem like the combination meter is the likely problem to me either considering how poor of a design it would be to integrate such a crucial transmission function into that part when there's already dedicated electronics for that.
If you really want to confirm you can get a used cluster for ~100 on ebay, and I can't imagine it would take more than an hour to swap that out. You could probably find somebody local with a parts car who would be willing to let you swap clusters for free in their driveway and only buy it if it solves your issue, especially if you buy something else from them at the same time.

What I'm getting out of this post is "never buy an older car with an automatic transmission". 6MT gang for life.