Yes it could be a thermostat. I would go head and replace it. Also I would flush that unierversal antifreeze out. I only use Zerex G-48 or get it from Mini. You can get Zerex from O’Reillys auto parts.

 

Yep. The Jarvik heart that Mini used for a T-Stat on the 2nd Gen cars can be particular about what's running through it. I used the premixed Mini coolant on my R56 after replacing the T-Stat. You've got to remember that these cars normally run hot anyway. Turning on the Max A/C should activate the fans and drop the temperature quickly. I know on my F56 that doing this will drop the temp from 235º down to ~190º in a matter of minutes. If that works, I would first flush the system and refill with Mini coolant. Anything to keep from having to dig into the engine bay to replace that part.

 

Yea what that vetsvette guy said.

 

Also, some codes will not clear on their own and are stored until it's cleared using the right code reader. I can't help you with that. Always had my Indy mechanic handle that stuff.

 

 

The coolant isn't causing the problem but the universal coolant is not good for minis. I use ether the MINI brand or the G-48, its the same stuff, in all three of our Minis. I am sorry I should have been more clear.
P0128 - Program map thermostat: Sticking in open position, As I am sure you already looked it up. I think you are correct in suspecting the thermostat.
But still I would flush that universal stuff out of the system.