Getting the same code P0128?

I keep getting the same DTC Code on my van, code P0128. I have changed the thermostat as well as the engine coolant temperature sensor. Code was cleared after each repair. I also made sure the cooling system and heater core was bled after the repairs. I did a pressure test on the cooling system too and I didn't notice any drop in pressure, yet I have to add coolant every couple of weeks or so. I do not notice any white smoke from the exhaust upon starting or driving. The van has never overheated, and the heater produces hot heat in a reasonable amount of time. The entire engine bay always smells of coolant, yet I've never seen anything leaking. I know I must be missing something, but I can't figure it out and this is driving me crazy.