This code came up for my car.

P0128-Thermostat opening too soon causing engine to run cold. (now fixed)

Would this be a possible cause for somewhat low mpg?

Just don't feel my Cavalier is getting the best mileage.

Average at 27mpg (mostly highway)

 

Colder the engine richer the mixture, MPG is directly affected by engine temperature.

 

If the engine runs too cold then the computer will always be in ' warm up ' mode and say that it needs more gas. The idle may be up as well.