cooling system P0128 code check engine light

[gto69]

Here is the problem I have right now with my 2001 GA. The check engine light (the one on the left side pod) comes on. So I used my OPD reader and had the code P0128 (coolant temp below thermostat regulating temp), One thing I noticed also when the light was on is that the radiator fans always keep running and the temp gage show 40 deg (look likes is not working).
So I check the coolant sensor resistance and is very high when cold and drop to around 400 Ohms when hot.

So I cleared the code to make sure of the trouble and as soon as the code was cleared, the fans stopped running and the temp gauge indicated the right temp (around 100 if I remember). I did that 3 times and it's the same story

So now what would be the miminum thermostat regulating temp that cause the trouble code and should I change the cooling sensor or thermostat cause I don't think of anything else I could check or change

Thanks