My f-150 threw a code P0128 this morning. I noticed on the way home from work that if I turn the fan all the way on high the temp gauge goes down to the 1st mark on the gauge ( the one that would be 1/4 tank if it was fuel). If I turn it off the temp would go up to almost half way. What would cause this. BTW the temp was 20 degrees F outside and my ride to work is about 60 miles. It has been doing this for a while (the temp gauge anyway) and I haven't noticed any problems from the truck's drivablity although I have noticed a 3 mpg drop.

 

Replace the thermostat and try again.