Thermostat and fuel economy

[briodan]

I have recently been getting a P0128 ODB-II code off my 2004 Grand Prix GT.
based on research i thought i would replace the thermostat, so i got a replacement one a OEM 195 i picked up at the local NAPA store. got a new gasket while i was there.
After replacing the thermostat i noticed that my fuel economy went way down, could it be the new thermostat?

[Fivefingerdeathpunch]

Doubtful.

I'd say you either need a tune up or its just the cold weather and winter gas doing it.