My 2004 E350 5.4L Van had a check engine light with code P0128. The most common repair is replacing the thermostat. Easy and inexpensive ($11) to do at home, takes less than a couple hours. Make sure to buy the right o-ring (gasket). Cleared the code and running great now with no check engine light.

P0128 code possibilities:
Leaking or stuck open thermostat
Low engine coolant level Insufficient warm up time
Faulty engine coolant temperature sensor
Engine coolant temperature sensor harness is open or shorted
Engine coolant temperature sensor circuit poor electrical connection