5th gear not working in 5-speed automatic

Question

I have a 2006 Mazda 3 Hatchback S with a 2.3L inline-4 paired to a 5-speed automatic trans. The car has just over 176,000 miles and everything works perfectly besides a sometimes sketchy idle and a transmission that sounds like someone left a loose nut jumping around with the gears at low and high speeds. At high speeds (highway mostly) the TCM will also randomly get confused, rapidly shift the car between 4th and 5th multiple times, then after violently lurching twice, it’ll throw the “AT” light and limit my car to only 4th gear. But it doesn’t limit my RPMs. According to other mazda3 owners and forums This problem is commonly caused by the TCM being mounted on top of the transmission housing which causes the computer to get hot and drunk overtime. The problem can be fixed by cleaning the TCM casing and moving it to a better location. Which is what I did. I have also recently changed the ATF fluid to full synthetic high-mileage ATF fluid AND replaced the output speed sensor yet the transmission still has problems with shifting unexpectedly and noise. I have looked into replacing the TCM with a new/used one but all records of other people doing that say that the problems occur again in about 6-8 months down the road and imo it’s not worth the $200-$400 for the TCM. I will say the car accelerates and cruises perfectly besides being 18y/o and a little noisy when the transmission isn’t acting up. Any input on this situation??? Thanks! I also really enjoy the videos you make!
Codes: P0720, P2187 (from $28 OBDll scanner)

scottykilmer · Answer

Well here's the thing, you have problems in a modern car like that. You will often lose your top gear because the computer will not allow it to shift into that gear. If you have some other problem, it's a software thing so that you don't damage anything. So basically, before you start analyzing the transmission, you would have to fix any problem. The vehicle had especially ones that are tripping trouble codes. Otherwise it'll never go into 5th gear. It may not be a transmission problem at all