This 2003 Buick Regal came in with a service engine soon light on and a code P0128 stored. The code means that the engine coolant temperature is below the thermostat regulating temperature. In other words the thermostat is sticking open or opening to soon to satisfy the computer. Thankfully this car has the 3.8 liter engine. The thermostat is very easy to change compared to the other engines. I started by removing the engine trim cover. With that removed the thermostat housing is easy to see.
I had already drained the coolant level down some. Then I removed the two bolts that held the water outlet (thermostat) housing in place.
I pulled the housing back to expose the thermostat. Popped the old one out and installed a new one along with a new seal.
I refilled the coolant system with the bleed screw in the water outlet (thermostat) housing cracked open. I started the engine and continued to open and close the bleed screw at different intervals, so that I could purge all of the air from the system. I checked the coolant level in the overflow reservoir, after the engine came up to temperature and rinsed off the coolant residue.