BMW car code erro P0128 OBD-II Trouble Code

i have a 2001
BMW 325CI 2.5 liter engine, the P0128 came on a few days ago and ireset
the code, and it came on again today. The water temputer gauge is at normal half way mark in the center of the gauge. I check the fan blade, and it is turnning. Can anyone help me with this problem, and what i can do to fix it.
What does that mean?

This means that the engine's PCM detected that the engine has not reached the required temperature level within a specified amount of time after starting the engine. The intent of the P0128 code is to indicate a faulty thermostat. Similar codes: P0125 In
determining the engine did not reach a "normal" temperature, it takes into account the length of time the vehicle has been running, the intake air temperature (IAT) sensor reading, the engine coolant temperature (ECT) sensor reading, and the speed of the vehicle.
Symptoms You will likely not notice any drivability problems other than the MIL illumination. Causes 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 Possible Solutions Past experience indicates that the most likely solution is to replace the thermostat. However here are some suggestions on troubleshooting and repairing
a P0128
OBD-II code: Verify coolant strength & level Verify proper cooling fan operation (check if it's running more than it should). Replace if necessary.

Verify proper engine coolant temperature (ECT) sensor operation, replace if necessary. Verify proper intake air temperature (IAT) sensor operation, replace if necessary. If the above items check out good, replace the thermostat If Nissan vehicle, check for
Technical Service Bulletins (TSBs), as the ECM may need to be reprogrammed to correct theP0128 code Other ECT sensor and circuit related DTCs:

P0115, P0116, P0117, P0118, P0119, P0125