Every winter on the first cold days my car throws the P0128 code! I am running a cooler thermostat I think {180 degrees} This winter its thrown the code twice so far. Need some opinions should I dump the cooler thermostat and go back to stock and flush the coolent while I'm at it. Also anyone got a trick to flush and refill the coolent

 

P0128 is for engine taking to long to reach normal operating temperature.
The colder t-stat you installed could be the cause for the code. I would try
replacing with a oem temperature t-stat.

It is true that a cooler running engine will make more power. That being said the gains are very small. The designers have set-up the engine to operate within a set temperature. Lowering the temp may lower fuel mileage & could cause premature engine wear.

You have nothing to loose switching back to the oem temperature t-stat.
Just my .02