Critter

They are calling for low temps next week, chances are your check engine light will again come on. With a faulty thermostat your call will take longer to heat up if the fault of the stat is that it is stuck open a bit or will not close totally. Fuel economy will be effected as the car takes longer to heat up and the computer will sense this and adjust fuel accordingly. Personally you could wait for better weather to change but do expect the same no heat or low heat in the near future.
Do not take the stat back, you will be changing it.