I am having the same issue. Temperature gauge seems normal and engine runs normal. I am curious how this original issue was addressed. X5 cooling system has multiple thermostat. Which one should I change?

one bolted to engine and sits between radiator hoses

http://www.autohausaz.com/secure/Par...1530139877.jpg

One sits below the expansion tank.

http://www.autohausaz.com/secure/Par...7111437362.jpg

dodoni1 listed a few probable causes, is there a good way to narrow down the faulty device?

A code P0128 may mean that one or more of the following has happened:

Low engine coolant level
Leaking or stuck open thermostat
Faulty cooling fan (running too much)
Faulty coolant temperature (ECT) sensor
Faulty intake air temperature (IAT) sensor