zXShadow

Hey guys, I've got a slight issue with my new rx8 that I need some help with. When I got the car, I was informed that the neutral safety switch was bad, causing it to have a jumping idle. I wasn't too worried about it seeing as it was the only thing wrong with the car. I've been doing some trouble shooting, I've got a CEL for p0850 and it does say there's an issue with the switch. While there's an issue with my idle when cold, through doing some troubleshooting here on the forums, I don't believe my switch is completely the cause of my idle issue. When it is warm the idle hunts from 700-900 and shakes when it jumps too low. I've read that pressing in the clutch would usually stop the issue if the switch was bad, but in my case, it does nothing and the idle still hunts. My cruise control also works completely fine, and I notice no difference when I am in neutral or not. My motor was rebuilt by T-REX motorsports in NJ around 10k miles ago. This is what i've determined so far:

-Compression is perfectly fine, not the issue, shows over 8.0 on all.
-Takes around 3 seconds to start but It's incredibly consistent and never does not start (i've got the old s1 starter, I think that is my problem)
-Idle will hunt slightly between 700-900
-no misfires
-zero issues when actively driving
-lights flicker ever so slightly when the revs drop to that 700 mark
-EXTRA poor gas mileage, like i'm talking watch the needle move bad, but I think its just normal thirsty rx8 stuff? Haven't had it long enough to know what the norm would be, plus I rev it out on the regular.
-My coils were done recently, but I've noticed one of them is held on with zip ties because the bolt snapped off too low to get a nut on, but its held on tightly and seems to work fine. Gonna replace with BHR coils soon.

I've also got another code that comes on after about 20 min of driving, flashing tpms light, says it has something to do with the ABS power module, but I notice no issues with my ABS. Maybe just a wheel sensor?

Necoxus

I haven't experienced the idle issue personally, however I've been learning the wiring diagrams for these cars for various reasons.

The MT cars have two switches that send signals to the PCM, the neutral switch like you mentioned and the clutch switch. There are two switches mounted to the clutch pedal. One is depressed in the pedal's resting state the other is depressed when you put the clutch in, the former is the one you're concerned with. I would check that switch out, see if it is visibly broken or see about getting a cheap replacement from a junkyard to test out.

Edit: the TPMS light you're getting is likely just a wheel sensor, the system on the RX-8 only checks for sensor function after you've been driving for a bit and I believe there is even a certain speed you have to reach. A OBDII scanner should be able to tell you which wheel has the fault.